20 sec sounds auwfully rapid for the viscous clutch to cycle on the fan. The fan WILL make a lot of noise when it kicks in--its job is to move a lot of air. It maybe the radiator core is mostly plugged and you reallyhave only a few tubes working. In that case, the fan will quickly 'see' cool air when it engages and will quickly cycle off. If you confirm it is the viscous fan, and not the electrics,I would raise the suspect level on the radiator to the top of the list.

Take it out and have it rodded out by a good radiator shop (a good one will have the crimping jig necessary to remove the top and bottom tanks. ) Cost less than $100.