Have a slight constant rumble noise from the rear end which seems to increase in volume and pitch on the overrun.  Local garage checked wheel & half-shaft bearings and said they were OK.  They suggested that I change the rear tyres which while legal were pretty worn.  New tyres fitted, but no difference.  Could it be the diff, and if so are there any oil additives I could use - obviously reluctant to shell out for a new unit. Also have a broken rear nearside spring so getting both sides replaced tomorrow, but I don't think that would be responsible for the noise.

Had exactly this problem on mine not long ago, Alex.

Initially the garage couldn't find fault with either rear wheel bearing, but that's precisely what it turned out to be - O/S/R wheel bearing.

Perhaps wait until the noise becomes more conclusive??

Check diff oil level. Filler plug on offside near the top. Oil should be level with filler hole