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.
Jacking it up and rotating the wheel is something I'll try tonight. It only does the noise when it's been driven for a while and it doesn't do it very long, so I'll need to be quick but it will indeed help me narrow it down.
It seems odd to me that it would be the belt rubbing on something, but you never know with these things.
