sjfehr

Failing AOS pulls vacuum on the crankcase- the squealing is air forcing its way through crankcase seals, which are not designed for high negative pressure. Blowing oil is obvious, but it can also make the most godawful stuck-belt-like screeching noise.

Good news is AOS is relatively cheap! If you're not willing to DIY it, at least get a new quote. $1400 is insane.

Dadio

Learn something new everyday. Not only was I unaware of the fact the AOS could make the squealing noise I would not have been able to explain why either. Good info to know. As for the DYI you mention, if it's the one I'm thinking of, it also includes a DYI on the alternator cable replacement which, given the access afforded by doing the AOS R/R , should be replaced at the same time. That job will save you an additional $1200 if you had to have it done at some later date. This is a common failure and has been know to fail in cars as recent as 2008. My 2007 suffers from the problem and I will be replacing it this winter along with the AOS while I'm at it.

911ct

997.1 C2 and C2S have different location for AOS(for C2 is on the driver side behind the intake manifold; for C2S is on the passenger side behind the AC compressor.

Dadio

There you go, learned something today that I didn't know. DYI I saw must have been on an S as it was behind the AC as you say.

Para82

If you're out of warranty i'd stay away from dealerships. Plenty of good independent techs can take care of that for a lot less.