My 4/4 had an axle ratio changed. Unfortunately it is well known that they “whine” a bit. So stuck some sound proofing material (metal sandwich sticky black stuff in the middle) under the cover board & down the front board. Plus both sides of the battery access panel. Then using carpet foam type underlay cut to same shape as the rear carpet. Finally the carpet. Whilst you can still hear the axle at some speeds with the roof (hard top)on. You can not hear it roof down.
