In one of my previous videos (I think it was the first one I posted today), you can see me check the pulleys for excess play; they seemed fine to me, but I'm hardly an expert.

It seems to me that I hear the noise the loudest underneath the car, where the gearbox meets the engine.

Belts and pulleys seem smooth to me. I'll take a short video and you can see for yourself. I'll check in the service records to see if the water pump has been replaced, but I think it's the original.

If you think you hear the noise more towards the rear of the engine than the front...

It makes me wonder why the belt might have anything to do with it.

I'd probably charge the battery, remove the belts, let the engine cool down until it is stone cold (overnight) and have someone else start it while I was under it, listening for the noise.

If you hear any noise, and particularly if you hear it as coming from the rear of the engine, I'd make another appointment with the shop and have them look for it, on a lift.

JR

Intermittent tick/chirp that goes away when belts removed sounds like either bad pulley, tensioner, or failing water pump.

BREAKTHROUGH!!!

I had the engine running with the belts on, and it was making the noise. Suddenly, I had an insight:

I turned on the air conditioning (which was somewhat noisy with the belt cover removed).

The noise disappeared.

I turned the air conditioning off again.

The noise came back.

I feel suddenly at ease that my engine isn't going to explode... but what does this mean? What's going on? Why would turning on the AC turn OFF the noise?

Well, when you turn the AC on you engage the compressor clutch.

Well, ok -- what does that mean for me? That it's probably not something catastrophically engine-endangering, right?

If the noise has to do with the AC compressor clutch, then no it's not life-threatening for your engine. You might have a problem with the AC compressor clutch, or you could have a problem with the belt tensioner, that changes when an increased load is put on the belt by the AC compressor.

That's great news -- thanks Javadog. I'll be sleeping considerably easier tonight.

^^^^if that's the case .......the A/C compressor most likely has a pulley bearing starting to fail. When the clutch engages it may stabilze & support the end of the pulley. When you remove the belt, carefully rotate the compressor pulley. It should be absolutely smooth with "0" lateral play.. Even checking this this way you still may not be able to feel or see the issue.

I checked the pulley and it didn't have much play -- maybe the teensiest, tiniest bit. I also looked to see if there's any wobble while belt is engaged. I dunno. It looks fine to me, but you guys can see for yourself. What do you think?

https://www.youtube.com/watch?v=FcDMvT0OjZk

Also, assuming that it is the A/C clutch that's the culprit (and I'm getting more and more confident that it is), what approach should I have to the repair? Is it a "don't drive it until it's fixed" kind of thing, or a "well, you can wait a week or two, but I wouldn't put it off indefinitely" kind of thing? Or somewhere in between?

Also also: what is the best way to KEEP IT from failing more quickly: to drive with the A/C off (which makes the bad noise) or drive with it on (which doesn't make the noise, but presumably puts more stress on the A/C unit)?

Hah! The m97 is a complex engine, but at least I don't have to worry about a broken warp drive!

According to a poster on Planet-9, "The AC compressor on our cars is clutchless variable compression. You may just be hearing the hydraulically actuated piston swash plate adjusting compression and returning to low." I'm not really sure I understand this exactly; anyone know what this might mean for me and my strange noise?

Is it poking good CJ? Compressors can make funny noises when they are known on freon.

Also, it's not catastrophic when it fails. Compressor will be smoked, depending on the method of the failure it will either free spin or lock up. If it locks up it will break the belt so you will lose and function driven by that belt. Most likely won't be able to drive far without that belt but no major damage done, unless it over heats of course.

All modern A/C compressors are variable displacement, but for sure there is an electrically operated clutch that engages it.

I'm going to guess that spell-correct had its way with this part of the message.

The AC is blowing cold, so I'm not sure it's the freon. Frankly, I haven't needed it much this year -- it's been a very mild (beautiful, really) Summer here in CT. In any case, I'll keep an eye on it.

my 987 aircon makes silly noises as well
It blows cool air since refilling, no complaints there
But sometimes make a sucking blowing noise in the front, and occasionally while driving
The compressor behind me makes weird noise.. i then turn it off a for a minute, and whisper sweet nothings to it "be still my lovely, take 5 minutes off"
It usually happens while sitting still in traffic.. at red lights or city driving, low speeds & revs.

If i turn it off for a couple of minutes the noise is gone.

It's not really audible outside of the car.