If your fan and motor mounts were relatively new, I'd say it's a bad water pump bearing. But really, bad motor mounts would have been my first guess if you hadn't replaced them so recently.
Also, I would think that even if the transmission mounts are the originals, the motor mounts should be able to keep things in check enough to prevent that much movement, so I'm thinking the tranny mounts don't have anything to do with it. I'd take your drive belt off and see if the water pump pulley has any slop. There's really only a few things it could be.
The only other things I can think of as even a remote possibility would be if you kicked debris up into the fan from underneath the car, or if your radiator was loose, etc. That stuff seems less likely, though.
Last edited by ZeGerman; 02-19-2011 at 08:12 AM.