I have a Honda Accord 1998 V6. There's this constant high-pitched steady squealing sound almost seems like its coming from under the engine cover near the serpentine belt. It happens whenever the car is idle or slowing down with my foot off the gas pedal. If I press the gas pedal the sound disappears. Its steady, almost harmonic. My timing belt was replaced at 170k 4 years ago. I only put on another 55k so far. So I had doubt it was the timing belt. But yesterday I had the car on and squirted some WD-40 on the serpentine belt in several locations but that didn't quiet or modify the sound, so this leads me to think its not a belt issue, as if it was it should modify or quiet the sound temporarily. I listened closely and it almost sounds like its coming from under the engine cover instead of near the serpentine belt.

This morning I noticed that the sound doesn't happen right away, but after 5 minutes of driving. Otherwise its always there when idle. Been like this for several months, but doesn't seem to affect the car or driving in any way.

Last year I think I overcharged my AC system so the AC was making a sound when on, but I had a mechanic let out some pressure, and it was fixed. Don't think that's directly related to this sound.

Any ideas?

 

You may want to start the car, wait until the noise appears, then use a piece of hose as a stethoscope to try to pinpoint the noise. Be careful with the moving belt.

After the noise appears, you may want to shut off the engine, remove the serpentine belt, then verify the noise is still there.

When the timing belt was replaced, was the water pump replaced?

 

As PA said listen carefully. Did you have factory parts installed? If not then it could be that the serp is bad, if so then it could be a bad bearing.

 

I tried using a stethoscope but didnt have luck pinpointing it.

It does sound like its coming from under the engine cover though.

I had the timing belt (and water pump, etc) replaced 4-5 years ago. It was probably not honda parts though. That's too soon for a replacement again right?

A couple other clues. It doesn't sound immediately after starting and idling. It takes at least a couple minutes before I can hear the sound start at idle or no-acceleration. Also, when moving fast and my foot is off the gas pedal, it doesn't sound, the high squealing, whistling sound only occurs at < 900 rpm or under 20mph.

I'll try use the stethoscope again, but it really did sound like its coming from under the engine cover instead of from the left side where the serpentine belt is.

 

Try removing the accessory belt and starting engine. You can run engine for some time w/o accessory belt.

If noise disappears, you know that an accessaroy belt-driven component is cause. Also while belt is removed, spin components by hand listening for a slight gravelly rasping sound. Bearing should spin easily w/ a slight resistance (grease). Idler pulley, AC compressor pulley, alternator are likely components to have a bad bearing.

good luck