The viscous fan ought to be silent in ordinary running and should only roar like an aeroplane engine when it is engaged to cool the hot engine.
Viscous clutches fail, usually, in one of two ways: Either the clutch seizes up and the fan spins at full power all of the time or the clutch becomes slack and doesn't spin the fan quickly enough at idle.
M103 engines had two different types in production: bi-metallic, heat, triggered and magnetic clutch triggered. The latter is obvious by the black electric cable running to the rear area of the fan clutch.