Mystery bearing noise from engine

[gsmaclean]

I've noticed a worn bearing noise coming from my engine (2003 4.6L) recently. It only happens at idle, and only when the engine is warm. When the noise occurs, the engine is bogged down and drops about 200 RPM. It will normally occur intermittently, for a few seconds, then stop for a few seconds, repeat.

The A/C compressor is not engaged.

With my mechanic's stethoscope, I listened to all the usual suspects - alternator, idler wheels, water pump - and didn't find it.

It sounds like it is coming from the left side, and believe it or not, the point where I can hear it loudest is the air filter box, particularly the bottom of it. I don't know if this is because it's coming down the intake and resonating inside the air filter, or because the air filter box is mounted to something that has a failing bearing.

Does this sound familiar to anyone? Any suggestions before I start tearing things apart?

 

[gsmaclean]

I took the engine cover off today and did a little more investigation.

The bearing noise seems to be originating from somewhere around the throttle body or intake manifold. It resonates down through the intake plumbing and into the air filter box, where it makes a kind of "howling" noise. If I take the air filter box off, it is "howling" quite loudly from the direction of the intake manifold. At the same time, there is a definite "bearing going bad" noise coming from the center area of the engine, right around the throttle body. I spent a lot of time with my stethoscope and was not able to locate it. It definitely does not sound like it is coming from the accessory drive at the front, and it's not coming from anything on that serpentine belt drive (alternator, idlers, etc).

It happens only at idle, and is intermittent. When it does happen, it bogs down the engine enough that it drops 200 rpm or so, and sometimes the engine will "goose" itself to recover from the drop.

Ideas, anyone?

 

[Carguy3J]

an IAC valve on the way out?

 

[gsmaclean]

Does it have rotating parts or bearings in it?

 

[Carguy3J]

Does it have rotating parts or bearings in it?

No, but it has a motor, and a moving "pintle". Minus the noise, it matches your other symptoms.

 

[gsmaclean]

You nailed it! I ran the engine up and waited until the bearing/howling noise started. I then unplugged the IAC valve, and the instant I did, the noise stopped. As soon as I plugged it back in, it started up again. Strangely enough, while the bearing noise is plainly audible to your ears when it is happening, when I put the stethoscope directly on the IAC valve, you can't hear it at all.

Thanks again for the help, I'll order a new one right away.

 

[Carguy3J]

It's not a bearing noise. What it prob is is air whistling through a partially blocked filter and/or passage in the IAC. Some people have been able to pull the filter off and clean it (the little round "knob" on the IAC. I've always either broken them taking it off, or not had the cleaning do any good. An IAC is cheap enough not to mess with trying to clean it. You've got the right idea, just replacing it.

Don't forget a new gasket, and clean the passages in the throttle body while you have the IAC off. Maybe clean the throttle body too, for good measure.

 

[gsmaclean]

Thanks, will do.