98 E320 while idling tapping sound appears to be coming from the power steering pump. Turning the wheel in either direction increases tapping sound at the idle RPM. Tapping goes away above 1000 RPM. All other indications
normal. Do these pumps have a history of failure. 52000 miles on vehicle.

To answer my own question, the belt tensioner is failing (it's next to the steering pump). Noise sounds like it's coming from the pump. Research shows the tensioner fails quite frequently on this model. Wholesale price $140.00.
Retail $260.00. Need Torx tools to change. Tensioner on order..