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I have a 2001 Honda Prelude SH (180k miles), I am trying to identify a whining sound coming from the right side under the hood. The sound consistently starts after I tap the throttle, lasts at a constant pitch for any where from 30 seconds to 5 minutes, then stops as suddenly as it starts: Video

The sound doesn't seem to happen within the first few minutes of turning the car on, only after the engine has been running for a few minutes (does not seem to be dependent on temperature; e.g. if I turn the car on when the engine's already warm is still doesn't happen in the first few minutes).

The sound has been going on for a few months. I can't pinpoint the source vertically, but it's definitely coming from the ps pump / alternator / ac compressor area (all three are on top of eachother in this car):

enter image description here

It doesn't seem to be the belts themselves (there are two, driven by crankshaft, one for the p/s pump and one for the alternator + compressor); the consistency of the sound's sudden start / stop seems to rule that out.

I don't think it's the compressor; the sound is independent of whether the a/c is on or not. I also don't think it's the p/s pump, it's independent of the steering state.

So I think it must be the alternator, since that's the only thing left. So my questions, before I start taking things apart, are:

  • Is that a sound the alternator can make?
  • If so, why does it start and stop and only happen after revving? What is the mechanism that is producing that sound (I want to know because I want to make it not happen any more)?
  • Could it be the belt? Maybe something about its tension makes it whine when the alternator is under load...? Does that happen?
  • If not, what else could it be?

I'm not sure if I'm having any problems with the electrics in the car. Everything seems to be functioning properly. The battery charges. It can handle full electric load at idle (I've tested with a/c + all interior/exterior lights + defrost on + stereo at max volume + an inverter charging a laptop while rolling all the power windows and sun roof open and close at the same time, has no problem, maintains a reasonable voltage).

There is an alternator test procedure in the service manual although I have not performed it. I don't want to start ripping out parts without a better theory.

If it is the alternator, I can't figure out why it would make that noise. I don't really understand how they work. The belt is always turning so I'm not sure why the noise wouldn't just always happen (assuming it's the alternator).

If it's useful, here is an exploded view of my alternator:

enter image description here

Is there something in the alternator that starts and stops spinning even while the input shaft continues spinning? I don't see any kind of clutch-looking thing in the diagram.

The alternator and compressor are the original parts. The steering pump... I don't remember, it's either original or at least 6-7 years old.

|improve this question
  • Where's the water pump? – Zaid Jul 17 '16 at 12:35
  • @Zaid Under the timing belt cover, runs off timing belt. In that engine image its sort of near the center on the right. I suppose if the noise is coming from there it could be reverberating in some weird way and misleading me, but I'm gonna assume for now that it's definitely coming from the red boxed area. Will be doing some tests today (with luck). – Jason C Jul 17 '16 at 16:24
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Try this: get a water spray and a friend that can help you to rev until the noise comes in, then start to spray one suspecting item, say, the alternator's pulley/belt. Just a bit of water there...and do with the rest if you don't see the noise disappear. What you will be doing is to lubricate the pulley with water, so the belt won't squeal. This is a very simple way to find slipping belt noises. Inside the alternator, there isn't anything that can mimic the sound of a squealing belt. Neither inside the a/c compressor. Those would produce other noises, aka, "grinding" because their bearings going bad...Other test could be if you unbelt the power steering pump, that will at least narrow the suspects.

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  • Thanks. Yeah the more I look around tbe more I also realize it must just be the belt when it has load on it, and I probably overthought this. I'll try that test (nice trick) and also check the tension tomorrow. I dunno why it's associated with the throttle. – Jason C Jul 17 '16 at 1:53
  • Can the pulleys themselves wear out somehow? – Jason C Jul 17 '16 at 2:03
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    Very unlikely, needs a lot of years to get worn out. I forgot to mention, a bad alternator bearing can eventually make high frequency noises, a bit similar to slipping belts...that's the step previous the grinding noise and happens for a very short time because the bearings gets damaged quite fast and get into the "grinding mode" in days. The spray water trick, trying to wet the bearings "may" work as well, if the bearing is really damaged and water gets in. – Aram Alvarez Jul 17 '16 at 2:08
  • Well I finally had a chance to play with this briefly; no love with the water test, but I'm still going to check the tension on those belts (it's a bit of a project, they're a little buried). If that doesn't work I'm going to pull both the belts off and see if that stops it, at least that'll help narrow it down. – Jason C Jul 20 '16 at 3:50

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