I have an odd situation I can't find solid info about online, and my Porsche-specialty mechanic is stumped. Can anyone here help?
Since December/January, I have had an increasingly prominent squeal/whine sound coming from the rear of my 997.2. Initially, it only occasionally occurred quietly when I actuated the clutch pedal. Now, it's louder, sometimes whines for short periods or cycles (wooo...wooo...wooo - here's a video) without clutch activity, and I can make it happen nearly on demand holding the clutch pedal about halfway through release.
2009 911 Targa 4, manual, 43k miles (19k miles until 2017 when I bought it, now a daily driver in western WA)
No CEL illuminated.
The clutch was replaced at the end of October due to pressure plate failure.
40k "major" service performed in mid-November.
My mechanic found that a crankcase vent line vibrates noticeably in sync with the sound. #2 on this parts diagram.
That line apparently connects to the air-oil separator, and he suggested the AOS may be failing (squeal caused by the diaphragm inside leaking?).
I found an article by Tony Callas noting that a failing AOS can honk and how to test it. My mechanic tested the crankcase vacuum and found it "within spec".
We replaced the vent line in case it had a leak. He thinks the sound is a bit quieter, but I don't notice any change in volume, and it still occurs as often.
Neither of us is keen to spend time and money replacing parts just to try them. I can't help but think the clutch replacement or 40k service is related, since this started not long after.
Perhaps the AOS really is failing? The Callas article claims 996/997 AOS failure often doesn't show smoke (unclear whether he means 997.1 or 997.2). Does anyone have more solid info on diagnostics?
I was once able to get a tiny rpm increase (50?) visible on the tach while holding the clutch pedal halfway on release while sitting in my garage. How might clutch actuation cause an rpm increase (electronic smarts to prep the engine for drivetrain engagement?) and is that even relevant?
Your thoughtful replies are much appreciated! I'd love to not spend $900 on the AOS without having a better idea what's happening.