Check the belt “guides” on both the bevel box and rear hub are not damaged. It sounds like the belt is rubbing on these.
If you jack the car up and rotate the rear wheel you should be able to see what the issue is the noise is quite distinctive.
and check the bevel box outer flange - your need a torch - from underside check the small button head retaining bolts for the flange - they sometimes start undoing and the bolt heads are so close to the swing arm they start to clip the swing arm - they continue to unwind and then get sheared off due to this action.....one by one, then the flange lets loose and wraps around the belt and you come to a sudden halt......hopefully not on a motorway...
