I went through that last year. Disassembled it and found a bad bearing. You can still get the parts to rebuild it, but its a PITA. I looked around and found rebuilt units from a number of sources, but they were expensive -- around $400 or so, if I remember correctly. Rather than breaking the bank on a rebuilt one, I ended up running a good used pump off of an E28 with some metric banjo bolts and custom -AN power steering hoses. It works well and I didn't really notice much of a difference in feel.