Hi iadom, thanks for your reply.
The belt is correctly seated around the drum and pulley and the drum turns with a little resistance. I have removed the belt to test whether the motor works when not connected to the belt, but it doesn’t turn. The noise is definitely a grumbling rather than a hum. There has been no trace of any burning smell. The noise doesn’t seem to be coming from the motor but from the other side of the machine and I wonder if it’s just the normal sound coming from the heater or condenser that wouldn’t normally be noticed over the sound of the motor and drum?