Back box wrong mounts?

I bought a mg zr 1.4 2004 i have noticed when im low down in the gears my back box vibrates on the rear bumper i thought it was blowing but i got my uncle to come over i revved the car up with my uncle behinde listening he pulled the back box down and it stopped. I looked at the last mot and noticed the only problem was the mounts for the back box im wondering maybe they put the wrong ones on ?? Is this a common problem ? The engine / clutch is strong best car ive owned i touch the throttle and the car is gone. Just need to sort this vibrating out...

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