I have a '99 328i (4dr - e46 model) and I'm hearing a jackhammer-like sound when i turn on the AC. I think the compressor piston is about to completely die. The freon/coolant seems to work fine as I do get cool air, but am hesitant to run it for fear of causing further damage. What should a repair cost me?

And if you live in the SF Bay Area, any repair shop suggestions?

Sounds like your AC compressor clutch is going out. You might be able to fix it now if you can find a shop that will rebuild the compressor using a new clutch. If you wait, it could lock up and throw a belt which could break other things. Your choice.

This probably isn't what you want to hear --- take it to a BMW repair shop ---

find it at www.google.com