The pulley is keyed to the shaft, but the fan is driven only by friction. Check the pulley nut, and I'll bet you find it's loosened up a little. Tighten it before you drive anywhere, at least a little, or you're likely to lose the nut and lockwasher, and possibly the pulley.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!