Q – My 2001 Boxster makes an odd whirring noise when I turn the AC on, I took it in and first thing the shop noticed was a loud idler pulley which they replaced, now they tell me that the AC compressor is the source of the noise.   I am not sure I trust them since they “guessed” wrong the first time.   What do you suggest? Pete

A – Serpentine belt drives can be tricky to diagnose.   These belts operate at very high tension and noises from one location can be transmitted to another.   Remember the “telephone that you made when you were a kit with two cans and a taut piece of string? This can fool well intentioned techs who simply call out the loudest noise location.   One was to tell where the source of the noise is coming from is to remove the belt and (with the engine turned off) feel the idlers and bearings in the various components such as the alternator bearing, the alternator clutch, the water pump, the idler pulley(s) and the AC compressor.   Unfortunately, when you are trying to detect a failing bearing by hand it is not being loaded and you are limited by that lack of tension. However, if a bearing feels rough or loose, it is a safe bet that it is not long for the world. A prudent approach would be to replace the obviously failing part first with the understanding that the sound could still be coming from one of the other components.

I would also suggest that you consider replacing the serpentine belt when you replace idlers since my opinion is that an old belt is far more likely to transmit noise than a new fresh belt.   (Perhaps one of you engineers reading this can explain this to me) I have seen a new belt cure noise on numerous occasions when the belt shows no outward signs of distress.   The added advantage of replacing the belt is labor overlap when doing the pulley.   This saves the aggravation of getting back into that area later.

The fact that the noise is only noticed when the AC is operating tells me that the compressor is the most likely source of the noise but that does not mean that it is failing.   It could just be low on refrigerant.   I would recommend charging the system after eliminating the idlers.  If the noise persists then I agree that a compressor may be the ultimate cure.   Clearly the shop should have informed you about ALL the possibilities before hand. MC