Squealing noise, need help identifying

987.2 Cayman S . There is a loud, intermittent squealing noise. It first happened immediately on startup, still in park. But it can be silent until warm, then the squealing starts. I can tell you what it is not. I first suspected the water pump, so removed the accessory belt and the noise remained, so it can't be anything driven off the belt. I also suspected the AOS and had a spare, so swapped that out and the noise remained. The pitch is not rev dependent, so is the the same nose whether idling or hitting redline, so seems to eliminate internal/drivetrain. It does not change pitch or volume whether stationary or moving, in P, N, D, R etc, so doubt transmission ( PDK btw). I can however, seem to influence the sound by pumping the brake pedal when stationary, and the squeal seems to either quieten down, or change pitch or even stop. I tried searching the forums, but as soon as you have "brakes" and "squeal" (or noise etc), you only get obvious issues relating to rotor noise. The sound is fairly loud outside the car, like everyone will look at you loud. It is not a quiet hiss, or hum. More like a whistle or constant pitch siren type of sound. Hard to tell exactly where, but seems to be behind the firewall, would guess front of engine rather than rear (hence my belt and AOS trials). It really doesn't sound like it is in the frunk area, so kind of thinking it's not the master cylinder . But then what is it?? Help! Anyone had this and can point me in the right direction? Thanks db

Have you checked the braking system vacuum booster ?

I haven't, but with the noise coming from the rear, I had kind of discounted the booster and master cylinder up front. Or did you mean the vacuum pump? That was my next target. I would have thought that revs would alter the pitch or something, but I guess if it a vacuum leak rather than mechanical, it may not impact the noise. Appreciate the reply.

If you can affect it by the brakes and it's coming from the rear then the vacuum pump would be my guess as well.

Thanks. Will see if I can reproduce it and put it on the lift to positively identify the source. Assuming vacuum pump, anyone have a replacement how to, video, forum thread etc to get me started? db

I haven't done it, but it looks like a reasonably simple R&R from below after you remove the rear underbody panel. Found this procedure online: https://f01.justanswer.com/tracker40/b94fe2ee-7789-4a85-9506-0b7bc46a1391_124.pdf

Excellent. Thanks so much. Looks fairly straightforward. No doubt there is the usual 1/8" clearance to fit baby fingers in there... Open to other ideas or suggestions from others. Will report back in case it helps others in future. db

before you replace the vacuum pump you may want to consider performing a smoke test as you may have a leak on one of the plastic/rubber hoses

A suggestion (and welcome to Planet-9 BTW..) - you're in the US, which means you're within 1000 feet of a Harbor Freight store. Go to HF, and look for the mechanic's stethoscope. It's usually $3.99, but sometimes it goes on sale. Since this sound isn't speed dependent that means it happens with the engine running, and the vehicle stationary? And it's not rev-dependent? Very odd. Rather doubt the vacuum pump since it's driven off the end of a camshaft and would be rev-dependent. One thing I've heard of that can make this sort of noise is a seal on the nose of the brake booster that points into the cabin. When that seal goes bad air is sucked into the booster (by the vacuum pump pulling a vacuum on the booster ) and the air going past the seal can make a squealing noise. That wouldn't be rev-dependent since the vacuum once achieved in the booster is pretty much constant. That's where the Harbor Freight stethoscope will come in handy. It will also be handy for lots of other noise troubleshooting in the future. It's a great $4 tool. Let us now how it works out, and please if you haven't - visit New Member Introductions - and introduce yourself and your car - pictures are very welcome there.

Thanks for the tips. I've actually been lurking on P9 for 10 years and found it invaluable. Most of my questions I have been able to answer through searches, but this one was odd (brake+noise=rotor/pad 99.9% of the time). Yes, that's in the plan. Correct. Noise is there only when the engine is running, but does not need to be moving, in gear etc. And given the brake pedal influence, it has to be part of the brake system chain (surely). Definitely not rev dependent, which is why I initially ignored the pump. But as I thought more about it, the pump should make (more or less) the same vacuum regardless of revs. So if it's the 'vacuum' part of it, then maybe I can see that. But if it's the actual rotating pump mechnism, I would definitely expect something to change with revs (pitch, volume etc). I'm reasonably certain this is coming from behind the firewall, towards the front of the engine. But could be the same concept at the pump end - small leak producing a wind squeal. It doesn't sound "metal on metal" type of squeal if that makes sense. And per Gilles, I plan to smoke test to see if there is anything obvious. I had done that maybe 6 months ago and nothing leaked. Thanks again. This is my daily driver, not a garage queen. And I love it - nothing else has caught my eye which is why I've hung onto it. Unless someone wants to swap for a GT4 RS, ha! db

Finally had some time to take a look. Did a smoke test - tight as a drum, the only leak I could create is from the smoke machine itself it was that tight (or the oil filler if I removed the cap). And of course it's not making a sound now so I can't even start poking around with the steth to find the source. I may take it for a drive around the block until it comes back then throw it on the lift while running to find it. Any other ideas? Thx db

So the drive started well - running like a dream. Then the squeal started. Of course the moment I pulled into our subdivision, it stopped. So still searching. I've left it running for 30 mins and it didn't return. It has appeared immediately on cold startup, so I know it's not a 'warm condition' issue, or vehicle moving problem. It is...random.

Update: so I haven't been able to reproduce the squeal (but haven't driven it either). When I started it today, there was a slight ticking sound that I felt was new. Yet it did not sound internal (or combustion at least). So I put it on the lift and hunted with the steth. Sure enough, it is the vacuum pump. It may just be wishful thinking at this stage, but I just didn't recall this sound before. It wasn't bad, just 'different'. So connecting to my earlier issues, I'm assuming (hoping) the pump has gone bad. Good news is I have a new pump. Bad news is that I have no idea how to pull it. I read the tech doc above (thanks @gubi ). All seems straightforward, but what they don't tell you is that there is about 1/16 clearance to the firewall. So I cannot even see or feel the top bolts. And even if I miraculously get them out, after looking at the new pump, it doesn't seem like there is clearance to pull the pump from the camshaft before hitting the firewall. So....anyone done this job? Any pointers? Please don't tell be you have to drop the engine. It would be a 5min job if dropped I'm sure. Thanks db

OK, so just a follow up and conclusion in case this thread helps others in future. It was indeed the vacuum pump. Although the concept of pulling the pump off and replacing it is easy (4 bolts, 1 pipe, slide off), the limited access makes this nearly impossible. I actually think it may have been bad for a while. Brake feel is much better and engine seems to run more smoothly. Most likely psychological, but makes me feel better.

So... Did you have to drop the engine to replace your Vacuum pump?