For months now a friend's '98 C230 (49K miles) has had a "GGRRRRR"ing noise coming from the right front end when the vehicle is turning left. Otherwise, steering response is normal.

The noise is loudest at speeds > 20 mph and occurs even when the engine is shut off prior to entering the turn.

The front end is properly aligned, right upper control arm has been replaced, right wheel bearings have been properly repacked and inspection of the remaining suspension and steering components reveals no gross faults. Steering noise was unchanged following the control arm and bearing service.

Power steering fluid level is full and no leaks are evident on system components. Installing a different brand of tire at the right front made no difference in the noise. Brake pad and rotor wear is normal; brake hose is positioned properly.

Any ideas as to what the cause may be?