I think your belt will run quieter when it is clean and wet, it probably will return to the normal noise very quickly if you have done nothing else, sorry to say. You can achieve the same by spraying silicone on it, for a short time. If your car has the NVH kit (post 2014), the belt tension should be fairly high as it is all rubber mounted and flexible, about 10mm deflection at the mid point. Much more than this and the belt will jump teeth on acceleration. If it is an earlier car with a solid mounted bevel gearbox it can be a bit slacker. Make sure you keep the rear wheel pointing straight in line with the chassis if you adjust the tension of the belt. Long term fix is a better rear pulley, i.e. not an alloy one.
