Diagnose Underhood Engine Noises
Is it a death rattle or just a bad alternator bearing you need to replace?
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.