e36's are notorious for leaking PS fluid... that's not really anything worry about if it is just seeping out slightly. I'm going to go out on a limb and say the e39's have the same problem. Honestly, I think you'd be best replacing the fluid and see if that fixes the problem. If it doesn't it could be numerous things. The steering rack, the PS pump are prime suspects and neither of them is cheap.
When I was at the track between one of my sessions I got a noise like yours. It went away almost immediately and came back on the first very cold day as a horrible groaning that went away immediately. I changed all the hoses and the reservoir and the problem never came back (they were leaking rather badly).