So, it seems that the radiator fan comes on and turns really fast and there's
 some sort of vibration (which could just be the speed). Also seems that
 normally the fan doesn't go that fast and doesn't make that much
 noise/vibration.

Ok, so as I picked up parts at my local shop (Wolf Sport in Berkeley), they
 suggested that this is a two speed fan, and it might just be jumping to the
 higher speed.

So my question is, what could be the culprit? Does the thermoswitch have two
 settings?

Any help would be greatly appreciated!

Thanks,

Philippe