When going 25 mph + and I turn the steering wheel there is a whispering noise coming from the engine compartment. This noise is not there when I turn the wheel when the car is stationary or moving fairly slowly. I particularly notice it on windy roads going 50 to 60 mph The power steering fluid level has not fallen and is correct. I have owned the vehicle for 3 years and had no steering issue before and not even topped up the steering fluid. The noise comes when the steering is turned just a little bit. There is no play in the steering and the car handles wonderfully. Any ideas what it is? What steps can I take to diagnose it?

Car: 1993 300D