I have a stock 2.7 liter 2001 Boxster with just over 35,000 miles on it. Last night when I pulled into my driveway after work, which is only 5 miles away, I suddenly got a loud squealing sound from the motor that I’ve never heard before. It kept on squealing and seemed to stop for a couple seconds if I revved the motor, but then resumed squealing at idle.
Because I had the serpentine belt and water pump replaced 650 miles ago, I suspected it might be the belt. I immediately searched this forum for posts about engine/belt squealing. An hour later, after I read several, I tried to remove the oil fill cap with the motor running. The motor did not squeal (after cooling off for an hour), and I did not run it very long or rev it. However, while it was running, there was too much vacuum for me to get the oil cap off, even wearing gloves and using both hands. The cap came off easily with the motor off, and I saw no evidence of collapse of the oil fill tube. (By the way, I never got a CEL or any smoke out the exhaust during all of this.)
Is the AOS the most likely problem? Any advice is welcome. Thanks.