I just had my local Mazda dealer install a set of diff mount bushings from Pettit Racing (street not track version) and now have a vibration from the rear when going between 40-50 mph. At that speed w/ or w/ out the car under power or in gear (not rpm related) the car has a slight but steady vibration. At any speed above or below 40-50 the vibration is gone. What could they have done wrong or is this something that was there but masked by the soft /worn out old bushing ?

 

Vibration? or gear whine?

I have the Unobtanium diff bushings and didn't notice any extra vibration, but did notice gear whine, esp on deceleration, but I like it