[Tatsfield]

If the water produces slippage between the belt and the resistant failing unit, the binding surfaces with momentarily stop moving against each other and the noise will stop. The problem with using this diagnostic is that if there is more than one unit being driven by the belt it will not tell you which one is grinding itself to death.

I don't know if the drive pulley is dual mass with a rubber buffer section bonded in between the pulley hub and the rim, but if that is the case, it is worth checking that the bonding has not parted whether there is slippage between the rubber layer and one of the metal layers making the sort of squeal that tyres make on shiny car park floors. The diagnostic application of water would also lubricate that and stop it squealing. A chalk mark across all three layers when stationary will no longer align if slippage is taking place. If the drive pulley is wholly metal, this will not apply.