Some info: 1999 E-320 (W210) with 66000 miles.

Question:
When I turn on the car a strange noise is made by the power steering system. The noise sounds like the purring a cat would make (Pbrrrrrrrrrrrrrrr!). It only happens once after turning on the car, lasts approximately one second, and does not happen every time (only after the car has been sitting for a while).
At first I thought it could be lack of Power Steering fluid, but the reservoir has got the right amount of Power Steering fluid. There are NO power steering fluid leaks and no steering abnormalities.
ANY IDEAS on what this could be??????????

yamil