Tap Coming From Engine

Okay so I drive a 2002 Subaru Impreza 2.5 RS with about 151,000 miles on it. When it runs I can hear a rapping sound sometimes, but it happens randomly. Usually when I step on the gas at low rpm but at high rpms it goes away. I don’t think it’s a spun bearing because I know what those sound like and they’re a lot worse. I also keep the oil in good condition and the highest I’ve revved it was like 5300 rpm and only for a second while the engine was warm. I also don’t think I spun a bearing because I’m able to drive it on the highway without issue and I think I may have thrown a rod by now if that was the case. What could it be?

Random rapping could be many things. Gassing it at low RPMs brings pinging to mind.

It’s hard to tell from the limited information, but if you’ve been using a lower octane fuel than the owner’s manual recommends… don’t. If auto-owner is right, and he may be, you can do permanent serious damage to the engine.

If not, you might want to let a reputable shop listen to it. See what they think.

And, if these are the original spark plugs, get them changed… ASAP.

I tend to agree this sounds like pinging you’re describing. In most cases this is caused by an EGR system fault. At 151k miles I suspect the EGR passages are sooted up.

Other less common causes could be a knock sensor circuit problem, heavily carboned up engine top end, or some hiccup in the PCM which is not retarding the timing when pinging is detected.

If this problem occurs more on a warmed up engine than a cold one then I suspect pinging even more.

Update: It tends to get A LOT quieter when the engine gets warm, I brought it to a shop and he said it’s probably the valve train making some noise and with the amount of miles on my car it’s time for thicker oil.

Exhaust manifold gasket leaks can cause mechanical tapping sounds.

Is the engine an overhead cam with shim-adjustable valve clearance? On those, if the valve clearance is too wide they can make a tapping noise which goes away as the engine warms up. If it’s non-adjustable hydraulic lifters, if one gets stuck it could make a similar noise. And it might unstick and go away as the engine heated up.

A little too much play in the connection of a connecting rod to the piston is another possibility.

I’ve had pretty good luck narrowing down the source location of a noise like that using a length of garden hose as a home-brew stethoscope.

Does this tapping occur at idle with the car not in motion? If so, then I’d tend to agree with a valve lash issue.

For what it’s worth, valve lash should be inspected and adjusted as necessary about every 30k miles. This is very seldom done by anyone and yes, I’m very familiar with the hokum factory recommendations about only inspecting it when hell freezes over or every other millinneum.