I have tried many of the helpful suggestions here. Thanks! I think it might be the belt, as it doesn't vibrate at all with the belt off. The belt isn't sitting in the small turning wheel on the motor, but sitting on the outside. I noticed that when I slowly turned the balance wheel by hand there is some resistance at one point of the complete circle. I'm just hoping it isn't any damage to the one plastic part in the interior arm! I don't think that can be fixed if it is cracked.
I also took apart the foot pedal (actually knee controller) and noticed that the machine starts vibrating once it hits an exact speed. Maybe I can loosen a spring and see if that helps.
It isn't the cabinet that is causing the vibrations. I took the machine out and tried it in my other cabinets, that I know don't vibrate, and the Pfaff still shook.
My Hubby has more mechanical aptitude than me, so we will be looking at it together this weekend.