I had a viscous clutch sieze up and make that same noise. Replaced it with the chevy express viscous clutch mod. Worked great ever since. Does still make the noise but you can tell it is not siezed as after I replaced it I no longer get the noise while driving down the road.
"That night we lay in our tent, looking at the stars and listening to the sound of the city. It does not sound like any other city at night. We hear heated discussions and a few fights. Also some gun shots." -Frederik Willems (While camping in the Democratic Republic of the Congo)