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Rear hissing "shhhh" noise at rear - ideas?

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Hi 996 friends,


I have a '99 Cabrio with 79K miles.  4 yrs now.


Just recently, I noticed at idle, or at any relative throttle, there is now a rear hissing constant "shhhhhh" noise that I can hear above the normal engine rumble when I'm inside the car.  It sounds like it's coming right from the other side of the rear bulkhead.  The noise gets progressively louder as the throttle increases.  When car shuts off, noise is gone too. 


The car's performance is unchanged, it is fully responsive in the full throttle range and at idle as it should.  The only difference is this new noise that has the same characteristic of like air constantly leaking.  The noise is constant and steady, no interrupted lulls or breaks.


When I try to hear the sound with the bonnet open and engine running, I can't really pinpoint where it's coming from.  I want to say from the middle to right side of the engine.  The engine noise from the outside clouds the "shhhh" , so it is most noticeable from the interior of the car with windows rolled up.


Also to note, there are no fluid leaks on the car or any other signs that could indicate a problem.  The only thing I did this weekend was have the convertible top changed, and when I drove it home from that it was fine.  Then on my next trip, I started to notice it.  Perhaps coincidence.


Anyways, thanks a bunch for any insight as to what the cause of this may be.  It surly has me scratching my head!!



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So I believe I know what the problem is.


I'm getting random idle fluctuations at stops,  constant hissing sound from rear bulkhead,  oil cap with car on has a rather strong vacuum when I try to remove it,  and there is some light white smoke coming out of exhaust when I start car for 5 to 10 mins.  I think the AOS is the culprit.  No CEL.  Thoughts? 



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Thanks JFP for your insight.  Yeah, it took me a little while to understand it.  I guess I totally had forgotten about the AOS.  Although I'm a DIY fanatic, I called my indie mechanic to get an estimate on what the total damage is going to be.  This job may be a little more than I want to handle, especially since I want a warranty on this in case for some reason this goes wrong the first time.  . All the symptoms are making perfect sense to pointing tot he AOS.





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