Here is a bulletin that may be your concern. Let me know if this is what you are feeling.
32 O3 97
Woodcliff Lake, NJ
This Service Information bulletin supersedes S.I. 32 03 97 dated October 1997 which should be removed and discarded from your S. I. binder.
The models affected have been expanded to include the 323i
Vibration or Buzz Felt in Steering Wheel
Some 328 and 323 owners may complain of a high frequency "buzz" felt in the steering wheel at a particular engine speed, and in some cases a vibration resembling a tire balance problem felt only in a two to five mile per hour window, usually in the 50 to 60 mph range.
This can be caused by hydraulic pulses set up by the power steering pump used with the M52 engine that are transmitted to the steering column through the steering rack.
A kit to stiffen the steering pump mount and to dampen hydraulic pulses in the system has been developed for cars that exhibit this condition.
In case of customer complaint, determine if what the customer describes as a "wheel shimmy" is actually being caused by the hydraulic system, by driving the car to isolate the speed at which the vibration occurs. Drive at a speed higher than the critical speed, drop the transmission into neutral, and allow the car to coast through the critical speed with the engine at idle. If the vibration is now gone, the kit should be installed.
Note :This repair does not apply to the 552 B32 engine used in the M3, or to the Z3.