I'm having the exact same problem as you described, I have a 330I / 2006 I have been the only owner, I know the in's and out's of the car, from all the noises to the individual smell. I started doing most of the work on it myself after the warranty expired and then had a bad experience with a mechanic. So a few months back I started to get this nasty squealing sound any time I would lock the steering wheel to the left. It's almost like the sound of belts slipping. Then I soon as I let go, the sounds seems to fade off. It's not the normal sound a PS pump makes, it's actually frightening. I started trying to diagnose the problem, but it seems to be coming from underneath the engine and rather not the pulleys, the belt tentioner, or belts themselves. I have yet to find the source. But just want to let you know that you are not the only one experiencing this problem, and I know that its not just the "normal" noise that the SP makes. Please let me know if you have found the solution to this problem, if I can't find it soon, I will have to take it in to the dealer.....