High Pitched whine - what might it be?

Last weekend I was driving on the highway and an engine speed related high pitched whine started coming from the back of the car. I was maybe 3-4 miles from my home when it started. I drove to the next exit about 5 miles further down the road and pulled off to head back home. I stopped at two lights on my way back to the highway. The whine diminished then went away as I went past the second light and started down the ramp. It did not return when I got up to speed. I turned around and went to my destination. Drove about 100 miles after that and the noise did not return. All gauges normal. Engine ran as good as ever. The car is a 99 996 C 2 coupe with 75k on it. All stock, IMS done. My impression was that it sounded like a failing ball bearing, perhaps in the alternator. Thoughts?

Does it occur only after the engine has had time to warm up? Does it occur only at low RPM/ idle? If so, your AOS may be failing and the sound you are hearing is air being sucked past the RMS. Had the same thing on my car a few weeks ago. Quite a few recent threads on AOS related events. You might find some useful information there...

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I'd say if the whine is not coming from your mother-in-law in the back seat, then it's probably one of accessories on belt line like water pump, tensioners, or alternator. Remove the serpentine belt and inspect the motion on all the units and belt tensioners.

At 75K, has your water pump ever been changed? Agree with above, remove serpentine belt and turn all components by hand checking for any play or roughness.

This is my first Porsche so I appreciate all the help. I got the car last October and am still getting to know it. I was / still am a 1974 MGB GT guy so this is all new. Car was up to temp by the time it started. I have some service records and these say the water pump done about 15k ago. New clutch and flywheel about 25k ago. IMS less than 10k ago. This so far has happened just the one time and was loudest at highest speed and less loud at lower speed. After I brought it down to idle at the traffic lights, and then sped up again, it was gone. Will read the ASO posts. Will also take off the belt to see if I find any roughness in a idler pulley bearing. Need to inspect it anyway. Thanks for the ideas.

No p-car content, but the alternator bearing went on my Mercedes.

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Sounds like a Click and Clack joke. When I bought my car, just a few months ago I heard the "death squeal", and was fairly certain it was a pulley, alternator or water pump. The first thing I did, after getting it home, was order all of that stuff for replacement. When the noise returned (after the work) I was very worried. As it turns out, I was worried about the wrong thing. I was certain it was the IMS bearing. I had it on the lift and the noise was definitely coming from that area. "Fortunately", it seems to be the AOS failure and crankcase vacuum sucking air past the rear main seal. Got a lot of great help and solid advice from guys such as yourself (Nutty). For the OP, I hope it is that simple (pulley or accessory bearing). If not - my saga is available at the "125,000 mile tune up" thread. Best of luck.

Check all of the pulleys - and check the inside of the serpentine belt to see if there is indication of fresh wear. Idler pulley bearings can seize...and my hazy recollection offers tantalizing hints of a 996 that suffered belt failure due to a dodgy AC clutch and another from a seized alternator.

Next time it does it, try turning off the AC and see if it makes any difference. I had an alternator bearing go in mine and you could hear it idling with the lid open. You definitely should be able to hear it with a cheap stethoscope (any auto parts or Harbour Freight). Same with idler pulley.

Could it have been that particular road you were on? Next time it happens, put the trans in neutral. If the whine pitch doesn�t change, the whine is either coming from the tires/diff, or part of the trans, or the secondary pump has kick on for the wrong reason. If the whine pitch changes with engine RPM, it�s not the tires/diff, but still could be part of the trans. At this point, you could pull over and stop in a safe place to rev the engine from 1,000-3,000 to see if the whine is still there, with or without putting the clutch in or out. I agree with the AC button check to see if that�s the cause. I would also try the fan/blower speed up/down/ and off to rule it out. Also try the radio, on/off and listen for the whine coming from the speakers. If the whine is at the engine, pop the rear deck lid and carefully listen for the whine on the right or left side, which could help point to what to check next.

AOS was my first thought when I saw your title...Good luck, it can be nerve wracking!

This is a key clue from OP - in that case it shouldn't be AOS since that whine doesn't depend on RPM. Most probably it's something rotational - accessory related. That's what I'd put my money on, anyway.

All helpful suggestions. I drove it Saturday AM and there was no squeal / whine. I could not reproduce it. I had to bring it in for a pre DE inspection and decided to ask the shop to check the belt and pulleys. Not the road by the way. Interesting thought. It is a rotational whine. I have an '95 Chevy 1500 WT and the clutch on the AC compressor made a similar noise 5 years ago. I turned off the AC on the truck and the squeal went away. Forgot about that till the post above. Maybe that's it, but the AC was on after it went away last week and on when I drove it Saturday to the shop.

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Always good to take a step back and take a deep breath. Nothing worse than shootin' at anything that moves with these cars. The bullets are lot more experience than grandpa's ol' chevy.

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In somethings in life, squealing and whining is what you want when it comes to reproduction. But in the case of a 996, you must keep a close eye and ear on it. But don't live in fear son, pull yourself up by the boot straps and go outside and tell the world, "you're not going to take it!" Tell'm you're not going to let the symptoms of a poor design rob you a little fun and excitement before you kick the bucket. Just remember, some days you'll be the *top dog* and then some days you'll be the hydrant. But don't let your circumstances affect your attitude. Cheers!

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Absolutely - and I think I have done that. Everything I replaced was due, if not overdue. So far it's holding steady, after a lot of research, analysis and action. So what you're really saying is - keep your stick on the ice. ..