Diagnose Underhood Engine Noises

You're tooling along on your way home from a cruise night or a car show when you pull up to a stop sign. As the engine drops down to an idle, you hear a strange sound from under the hood. It's not a good sound either. Your heart sinks as you realize the sound most likely means expensive internal engine problems and you're going to have to take your Mustang off the road, not to mention spending considerable cash to make it all right under the hood so you can enjoy your Mustang once more.

Hold on though, all may not be lost. The sound you're hearing could simply be a bad alternator bearing--a simple fix that'll get you back on the road in an afternoon. However, that sound could also be coming from deep within your engine and mean you're about to get elbow deep in an engine swap or rebuild. So how do you tell the difference? That's what we're going walk you through in this story.

We recently had an engine noise, a metallic chirping sound, rear its ugly head under the hood of our '90 Fox-body. Intermittent at first (the worst kind of noise is the kind that doesn't happen all the time!) and hard to pinpoint, the noise came and went over the course of several weeks, finally reappearing and staying with us for three solid days. It was at that point we knew we had to get under the hood and delve deeper.

Using a mechanic's stethoscope, the noise was pinpointed to the No. 2 cylinder area. The sound was also determined to be at the top of the engine and based on rpm (we'll get into how we did that shortly). After breaking out the tools and pulling the valve covers, we found excessive wear on the roller rockers. Mind you, this is a 347 stroker with over 120,000 hard-driven miles. We bolted up a fresh set of rocker arms, but upon restarting the engine, the noise was still prevalent. Going back in, and deeper this time, we found failed lifters and damaged cam lobes, including a bad cam bearing. Our day was done and the engine would have to be pulled for a rebuild or replacement.

Hopefully, your engine noise won't be as grave, but if so, you'll at least know how to pinpoint it with these instructions. Feel free to share your engine horror stories with us on our forums at www.mustangmonthly.com!

Identifying the Noise

First and foremost in engine noise diagnostics is identifying the noise. Is it a metallic sound or perhaps a ticking sound? Maybe it is a knocking sound. Whatever the sound, you have to determine its location, operating condition (is it based on rpm, change under load, and such), and at what rate the sound is happening. We can start narrowing the noise down by two primary categories/locations. Generally, an upper engine noise issue will be a ticking sound, while a lower engine noise will be associated with a knocking sound.

Upper Engine Noise

For upper engine noises related to valvetrain, the noise will happen at camshaft speed, which is half the speed of the crankshaft or the rpm indicated on a tachometer. For example, if your engine is idling at 600 rpm, the camshaft is rotating at 300 rpm. As such, a valvetrain noise will occur five times every second at this rpm. Valvetrain parts that can be sources of such noise include the camshaft, lifters, pushrods, rocker arms, springs, guides, and the timing chain. Noises associated with the upper engine and valvetrain will increase in frequency with rpm, but will still be half of the engine (or crankshaft) rpm.

Lower Engine Noise

Lower (or bottom end) noises, generally appear as a knocking sound, deeper in tone than something from the top of the engine. The bottom end noise will occur once per engine revolution. If your engine is idling at 600 rpm (and therefore that is the crankshaft rpm), the sound will occur 10 times per second. Lower engine noise sources include the crankshaft, connecting rods, pistons, main and/or rod bearings, flywheel/flexplate, and various excessive clearances such as a mis-bored cylinder causing piston slap. Like upper engine noises, lower engine noises increase in frequency as rpm increases, but they can also be affected by engine load and temperature as well. A knock might be heard in gear under load, but not in park at idle.

Load and Temperature Variables

As we just stated, engine load can vary the intensity of the noise. Sometimes load will make the noise appear, and other situations it will make it go away. An old trick is to pull the spark plug wire off of a cylinder in question to remove the load from that cylinder, lessening the pressure and usually reducing or eliminating the knocking sound during diagnostic testing. Temperature also plays a vital role in engine noise diagnostics. Noises associated with excessive clearances will often go away once the engine is up to temperature and the metal parts have expanded. Sounds based off of the oiling system (bearing clearances, and such) often do not make noise when cold, but will once the oil has warmed and thinned out. These are all diagnostic tips to keep in mind while checking out your engine for noise issues.

Where to Start

We all hope that a noise coming from under the hood is going to be a small and inexpensive repair. That may not be the truth in most cases, but we can keep a positive outlook until we know for sure where the sound is coming from!

Using a simple mechanic's stethoscope (or even a long screwdriver with its handle nestled into your ear), place the tip of the tool on the accessory drive items first. Listen to the alternator case, water pump housing, power steering pump housing, and so forth. For you late-model owners, check the spring-loaded belt tensioner as well. Be mindful of spinning engine parts, but get in there and check everything. You'd be amazed at what a bad water pump shaft bearing sounds like.

Amplifying the sound with a stethoscope or screwdriver allows you to easily pinpoint the location. If need be, have a friend control the throttle to determine sound pitch changes to the offending part/bearing. If your accessory drive checks out okay, then move rearward, checking components on the way. Exhaust leaks can sound like valvetrain ticking, so be sure to listen around collectors, gasket surfaces, and more. Loose sparkplugs can also cause a metal-to-metal clicking sound due to the combustion chamber's pressure rise and fall, causing the loose plugs to rock in their threaded holes. Listen at the valve covers for rocker related noise. You may even have to get under the car (while it's running) to listen at the bellhousing for a loose or cracked flexplate, or damaged clutch. Remember, safety is your number one priority while performing these checks.

Digging Deeper

So let’s say you’re having as bad a day as we were with our 347 and all of your accessories check out fine by making their normal mechanical noises, and you have no exhaust leaks creating a false valvetrain ticking sound. Your noise is either at the top or the bottom of the engine, and with your long screwdriver or stethoscope, you should have zeroed in on the location a bit more precisely. We’ll walk you through both top and bottom diagnostics so you can get to the source of your noise.

For top end/valvetrain noises, you’re going to have to remove the valve/cam covers (and this means the upper intake for you 5.0L EFI Mustang owners). If you can run the engine with the valve covers off (carbureted, for example), all the better. For those with EFI, you’ll have to rely on visual inspection or possibly use the starter to spin the engine. Cover the engine bay with an old towel or shop rags, as running the engine with the valve covers off will cause oil to splash everywhere. With the engine running, use the wooden handle of a hammer to apply pressure to the rocker arms, both on the valve end and the pushrod end in an attempt to lessen or eliminate the noise. You’ll need to do this for each rocker arm.

If the noise is lessened or eliminated when pressing on the valve end, look for issues with the valve, guide, spring, or rocker arm. However, if the noise is lessened or eliminated when you apply pressure to the pushrod end of the rocker, then look to the camshaft, lifter, pushrod, or rocker arm for the source of your noise. Your issue will fall into valve clearance/guide problems, a broken valve spring, loose or worn rockers, a lack of lubrication (look for discolored metal), worn lifters or cam lobes, or a bent pushrod. If a visual inspection doesn’t catch anything, you’ll have to start disassembly to search for the problem part, although the investment in a boroscope or digital inspection camera will help find problems in the combustion chamber, ports, or internal engine much easier without a full tear down.

Lastly, don't forget the timing chain. Wear here can cause contact with the timing cover, causing a noise. Be aware that some timing chain noises can sound like a bottom end rod-knock too.

Moving to the bottom end, things can be more difficult as you can’t always access the parts as easily as the valvetrain and some of the sounds failing parts create can sound like other areas of the bottom end. As in the top end inspection, your first check should be a visual one. Inspect the damper for shifting, sheered keyway, or loose retaining bolt. On automatic cars, drop the inspection plate and check the torque converter retaining bolts/nuts to ensure they’re tight and not rubbing on the block plate, or have loosened and slotted the flexplate mounting holes. Remove and inspect the starter drive and nose housing for loose or broken parts (this includes the ring gear on the flexplate/flywheel). While you can’t really see the flexplate attaching bolts, you can often use a pry bar to gently pry the flexplate. A good flexplate has little movement, where as a loose one, or one with cracking, will shift easily or make a popping sound as you pry against it.

Sounds associated with flexplate/converter or flywheel issues include a rod-knock type sound due to cracking in a flywheel. This sound is most noticeable on acceleration and changes on deceleration. You can check this by driving at a constant 20 mph and then shutting off the ignition. If you hear a thud, then the flywheel/flexplate is suspect. Loose torque converter bolts/nuts will create a knocking type sound at idle or when there’s no load applied. If you blip the throttle in park or neutral, you might hear a quick succession of knocks. Sometimes a converter/flexplate/flywheel sound will happen in gear but not in park or neutral.

Internal engine noises that come from the crank, rods, and pistons are a whole different issue all together. As mentioned before, it is extremely difficult to inspect these areas without major engine disassembly. Vintage guys are lucky in that it is easy to remove the oil pan and inspect main and rod bearings and hard parts, whereas the late-model guys have the huge engine crossmember to deal with. The double-sump late-model pan is difficult to remove without at the very least removing the upper intake on the 5.0L and raising the engine off its mounts as high as you can. Modular engines are even more of an issue and generally require complete removal for tear down type inspections.

Piston noise is usually caused by one of three conditions: excessive piston to bore clearance, excessive piston cross-pin to bore clearance, or excessive rod bearing clearance. These issues usually emanate from an incorrect bore sizing, incorrect piston sizing, or mis-shaped or machined pistons. A piston to bore clearance issue causes what is known as piston slap. This noise is usually most noticeable at cold start up and will decrease as the engine warms and the pistons expand; sometimes completely going away. It is fairly common with forged pistons, but can also happen with cast or hypereutectic slugs as well.

When a piston pin has too much play in its bore, the sound is often described as a double knock at idle speeds. Rod bearing issues rear their heads as a load sensitive knock, with the intensity of the knock increasing with load (rpm). A rod knock can usually be pinpointed to the front or the rear of the engine, but direct inspection of the rod bearings will be required to find the offending part. Additionally, cracked pistons, broken ring lands, and incorrectly installed pistons will all create some sort of noise. A boroscope through the spark plug hole is a simple and effective way to find these issues.

Lastly, we have crankshaft noises. The typical crankshaft knock is hard to pinpoint, and sometimes will even disappear depending upon load, temperature, and other factors. The only way, once again, to be sure of your crankshaft’s clearances is to drop the pan and remove the main caps one at a time for a visual inspection and to verify bearing clearance measurements using Plastigauge.