Jakob E.

Hello -

As the title suggests, my 2007 911 C2S is making a very strange squealing noise that I have yet to find the solution to. Yes, it is probably off to the mechanic for a once over, but I wanted to see if anyone had some suggestions here first. Let me preface this by saying this noise first occurred when getting off the interstate and sitting at a stoplight. It comes and goes randomly, I see no pattern with temperature (fluids or atmosphere), rpm, or anything else. When I made it home, I immediately started listening to pulleys and disconnected the accessory belt to rule out that, all good. So then having done this before, I thought maybe it was the AOS, bingo. Tons of vacuum on the filler tube and oil at the throttle body.

Having replaced one before, I DIY'd the AOS with the help of forums and the shop manual. Rather than just the main unit, I decided to do the whole kit, so both secondary AOS units and all the hoses (Vacuum and Coolant) with Genuine Porsche Units. Due to the mileage (203k miles), I decided now would be a good time for an injector replacement(Genuine Porsche Parts), maybe overkill, but I was in there so why not. Cleaned the manifold and plenums throughly of oil and buttoned it all up. Noise is still persistent, but no vacuum at the oil filler tube. I checked in hopes maybe the new unit was faulty. So what I know, the car definitely needed a new AOS unit because of the vacuum pressure and oil in the manifolds, but why does the noise persist? All units were torqued down using the Porsche 997 workshop manual specifications.

My guess is the air injection pump relief valve, but I would rather not take everything back apart without a clear direction. The car continues to run like a top with no loss of performance or any codes. Just a terrible noise that from what I can tell is biased to the left side of the engine (video attached). The video is about 30 seconds after starting the car which is when the noise starts and it can get quite loud, but again with no clear pattern. Sounds like a tea pot whistling away. The noise in the video is faint, but sometimes it really gets going. About the only place you can hear it on video is in the left rear wheel well, so the video is uneventful.

Some final pieces of info, the car is serviced every 5k miles with Motul 5W-40 and a new filter. Air filters are replaced annually. It has new coolant running through it. The power steering reservoir is filled with fluid and the rack/pump are both in good working order (both fairly new). The tip tranny has been serviced and presents no issues. The car never overheats and has extremely strong oil pressure, so this is not an internal motor issue in my mind. Spark plugs and coils were replaced just 5k miles ago (Genuine Porsche Parts).

I am stumped and appreciate any feedback.

-Jake

jchapura

You might try smoke testing the throttle body, intake. Another similar method is to use a flammable carb cleaner and, while engine running, spray it gingerly at airway parts and connections and listen for changes in idle. Take safety precautions, of course.

Also, temporarily remove your serpentine belt and see if you can reproduce the bad behavior.
​​​

Jakob E.

I did isolate the serpentine belt out of the equation, so we are good on that front. I will see about doing a smoke test for sure. And will report back.

-Jake

Petza914

I don't see a link to a video or attached video with the sound you're looking to resolve, but it seems like you've already addressed just about everything that could create that.

Jakob E.

I attached the video link this time, hopefully it works. The sound comes and goes randomly. The volume of the sound also fluctuates. It sounds textbook AOS and presented all the symptoms, but after a total replacement, I see no reason to believe the unit is faulty based on checks run.

Petza914

I don't hear any type of squealing or whistling sound in that video. There may be a higher frequency metallic ringing type sound - is that maybe what you're talking about or is there a better video where the sound is more pronounced?

Jakob E.

That is the trouble with sounds, I guess. No other video to capture the sound any better. Had someone else take a listen and they said it sounds like a blender. So let us assume it is the high pitched metallic ringing that I cannot accurate describe. Thoughts?

Canada911S

I’m pretty sure I’m having the same problem. The noise shows up after driving for a while. Does it subside if you apply slight pressure to the left tail pipes? When I do that, the sound stops.

Jakob E.

The noise I experience is pretty consistently happening from startup to shutdown, but does go away randomly. Or at least I stop being able to hear it. The car is booked for a checkup
August 14th, so I should have more answers soon enough. Have not tried apply pressure to the exhaust because the sound is from the top of the engine, but the wheel liner is the best place to capture it on video. Less hoses and no manifold in the way. I suspect your issue could be an exhaust leak or PSE vacuum line leak (if you have it).

Jakob E.

We have an answer: The mechanic at first thought it was the pulleys for the serpentine belt, so they suggested replacing them all with a new belt too. This was not the answer, but I was going to replace them this winter either way because of their age, so one less thing to worry about. Also, I isolated this out of the equation initially. The noise in fact was a vacuum leak from the control valve for the coolant circuit of the transmission. The valve was not leaking coolant and again no codes on the car, but the diaphragm inside was allowing air to "squeal or whistle" past. When the part fully fails it shuts out the oil cooler from the coolant circuit, but I got lucky. Cheap part to replace and easy to access, so all in all not a bad job. Mechanic said he has seen them leak coolant, but never make this high pitched noise.