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Screeching noise coming from back of car

I was driving my 996 Coupe on the freeway in the rain a few days ago when I heard a screeching noise that kind of came and went. I couldn't tell where it was coming from and thought the noise might be from an older car behind me. It sure looked like it could have been.

The problem was that the noise never entirely went away, but the car never did leave the location to the rear.. I didn't smell anything from the car and, figuring it wasn't in mine, kept driving.

After about 20 miles, I had to take the exit ramp to our house. When I got to the bottom of the ramp, I could hear that the noise was from my car. I didn't have much further to go so went ahead and drove home and parked it in the garage. As I drove the noise came and went. I checked the gauges as I drove, and none of them were out of normal range. That included oil pressure, coolant temperature, and so on.

Occasionally, the idle would bump up to 1200 RMP and then drop down, but most of the time it was right at 800 RPM.

After letting the car sit for a day and a half, I went out to start it, and light gray smoke poured out of the back of the engine compartment. There was no noise coming from anywhere this time, and the car idled well before I quickly turned it off.

I'm curious as to what you all think might be the problem.

I pulled the serpentine belt and checked the pulley on all of the parts it drives. None of them showed any sign of abnormal spinning. They were all on tight including the tensioner and the upper and lower idler pulleys. The air conditioning compressor spun freely, continuing to spin after I stopped rotating it. (I don't know if that's normal.)

Could this be a problem with the AOS or would that show up as a Check Engine Light? Nothing has lit up at all and all of the values in the gauges are still normal.

Thanks for your help.

Monte
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Old 05-15-2017, 05:29 PM
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Was the smoke coming from the exhaust? Or from around the engine compartment?
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Old 05-15-2017, 07:09 PM
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It could be a loose spark plug that is making a whistling noise. Your screech description is subjective and could mean anything. A worn belt, idler pulley , etc as you mentioned. If it sounded more like that than a whistle or whine, then keep checking. If it could have been a high pitch, check the plug tightness.
Did it raise and lower with rpm?
Old 05-16-2017, 06:42 AM
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Quote:
Originally Posted by looneybin View Post
Was the smoke coming from the exhaust? Or from around the engine compartment?
It appeared to be coming from the engine compartment and not the exhaust. It happened very quickly and pretty much filled the air around the car with smoke. Hope that makes sense.
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Old 05-16-2017, 10:49 AM
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Quote:
Originally Posted by tonythetarga View Post
It could be a loose spark plug that is making a whistling noise. Your screech description is subjective and could mean anything. A worn belt, idler pulley , etc as you mentioned. If it sounded more like that than a whistle or whine, then keep checking. If it could have been a high pitch, check the plug tightness.
Did it raise and lower with rpm?
Hi,


The pitch stayed constant but it would sometimes completely disappear at lower RPM.

I appreciate your thoughts and time on this. I know how hard it is to troubleshoot a problem from a written description. I may try to go out and video it if it pops up the next time I start the car.

I was even wondering if it might be the Air Oil Separator, given the smoke that poured out with no idiot lights coming on.

Thanks for your thoughts.
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Old 05-16-2017, 10:52 AM
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By the way, I checked all of the pulleys, and they are solidly on the engine. No grinding or other things going on. The serpentine belt is nearly new.
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Old 05-16-2017, 10:54 AM
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Second vote for loose spark plug but doesn't necessarily explain smoke. AOS is typically out of exhaust.
Old 05-24-2017, 07:40 AM
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AOS is my guess. Have someone stand where they can see the exhaust pipes and start it to see where the smoke is coming from.
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Old 05-29-2017, 03:34 PM
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I experienced similar sound from my '02 C2, but never any smoke. It turned out to be the secondary air compressor which kicks on periodically hence the intermittent noise. Might be worth checking on, you can do that by disconnecting the power leads (with engine off of course) and applying power to operate the compressor. There is a tech article on changing this here in Pelican. I purchased the replacement through Pelican at substantial savings vs dealership and replaced it myself. Good Luck!
Old 06-22-2017, 12:43 PM
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AOS. Check it out. When you shut the engine off the sound will continue for awhile until the pressure within the engine bleeds off.
Old 06-27-2017, 09:36 PM
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Open the engine lid and let the kitty out?
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2004 996 Turbo CAB X50
2003 996 cab 6 speed Sold
1972 RS 3.2 twin plug short stroke crank fire, roll bar, sold
DE instructor since 1985
Frank
Old 06-29-2017, 04:00 PM
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