AFAIK, the fan pulley is fixed as are the pulleys next to it. The only possible contact is between the tensioner pulley and the water pump pulley. At the point those pullies touch, however, your belt will be too loose to drive anything.

I don't know what sound you're hearing but I'll guess it's not the belt slipping since it would be dry by now. My guess is that water got to your alternator bearing since it's positioned the lowest among the belt driven components.

Good luck,
Sixto
78 450SEL
91 300SE ... with belt problems of my own