1998 ML 320, 99K miles, developed a constant power steering whine, although the PS fluid level was correct. The whine was clearly audible within the passenger compartment, not temperature dependent, increased in intensity as the engine speed increased and changed little when the wheels were turned left or right. Outside of the noise, the PS system operated flawlessly. Inspection of the hoses and rack revealed no evidence of fluid leaks or kinked/collapsed hoses.

The PS oil was renewed with no effect. The pump was then renewed but the noise is still present, although the intensity is lower. Fluid drained from the system at the time of the fluid change and subsequent pump change was clean and free of particulates. The fluid reservoir was also inspected and found to be free of debris. Hoses are firm and not spongy. The noise is clearly pump related... it disappears when the engine is run with the poly-V-belt disconnected. Again, other than the constant whine, the PS system appears to be operating properly.

Is the new pump defective... or is something else amiss? The dealer advises they sell a fair number of these pumps but doesn't recall seeing a bad one out of the box.

An obstruction within the system... perhaps the rack? If so, how does one locate and confirm an internal obstruction when there are no outward clues as to its location? Replacing hoses piecemeal is one thing but rack renewal is too expensive to perform based on just a quess.