Steering Wheel vibrate at certain RPM between 1200 - 1400
 

I recently bought an Audi A4 1.8T quattro 2002. It runs good at hight
speed. But I can clearly feel the vibration from steering wheel during
acceleration and deceleration when RPM hits the range from 1200 to
1400. Below or above this range, the vibration is gone. And there is a
not loud but noticeable humming noise coming with the vibration. The
vibration can barely be felt from the seat or clutch, only from
steering wheel.

Sometimes it is really annoying so I went to 2 dealers. The first one
changed my catalytic converter and an oil valve. The other one changed
4 engine mounts for me. But none of this treatment solved the problem.
I have my car checked thoroughly by them both. There is no leak in my
car.

I just wonder could it be some problems with my engine or
transimission? Does it sound like a serious problem? What could it be?
I have doen some internet research. There are 3 major opinions on that:
[1] The timing belt may be loose. I had it checked by dealer. No loose
timing belt is found. But I heard timing belt is really hard to check.
Maybe he didn't check carefully for me. Is there any way that I can
check the timing belt by myself?
[2] Front-end balance, steering balance or motor mounts. I had all 4
engine mounts changed by the second dealer. Nothing improved. Is there
anyway that I can check the balance by myself?
[3] Power steering problem, power steering muffle or something else. I
don't know yet. I got my car checked by the dealer but no specifically
on the steering system. No problem was mentioned by dealer. Could that
really be the problem for my situation?

Every time I went to dealer, I have to take at least half day off. I
don't want to waste my time any more. I am planning to go there one
more time. But this time I would like to bring some knowledge with me.
So I need your help to give me some share of your experience or
technical knowledge. Any share will be warmly appreciated.

Michael Wei