Tyre noise vs gearbox

Back to the old bearing noise just had a garage suggest its a gearbox issue due to the rough grinding at 20. It then goes quiet and bearing noises appeare over 50, another mechanic friend has shown me that the inner edge on all 4 tyres have castllated and may be the bearing sound any ideas, don't want to put tyres on as they have over 6 mm tread and 1000 for a gearbox is too much at the mo