1995 Mercedes Benz S500 power steering pump.

My benz has developed a noise problem when cold. The power steering pump whines and the steering is noisy when I turn the wheel back and forth. The noise stops when the engine heats up to normal temps. The routine is to heat up the engine, turn it off for 5 minutes and them turn it back on. The steering stops making noise and the whine in the pump stops.I changed the fluid and filter and the original return lines(2) but that didn't help. The fluid level doesn't drop or there does not seem to be anything leaking other than a little around the front seal. Help, help, even in South Carolina it gets cold in the winter.