Essentially you are getting air into the PS system from somewhere. Normally this comes from the high pressure side of the system, so the two main things to look for are the high pressure hose and the pump.

I've had this problem for some time (on a 1989 530) - it got really bad last year with the steering groaning all the time. So I replaced the high pressure hose (the expensive one) and that fixed the problem of groaning in the summer. I still have groaning on cold (freezing) mornings so the next job is to replace the seals in the PS pump. These are pretty cheap about 25 USD from pskits.com (shipped them to me in the UK with no problem) and there's a nice video somewhere on the internet showing you how to do it.

I'm now waiting for some warm weather to do the seals ...

Dermot