Join Date: Jul 2004
Location: M-F: Thailand Weekends : F1 2010 - Various Tracks!
I think it's hitting the enclosure.
Perhaps the fan flexes when it spins up and that is enough to foul any clearance in it's enclosure.. It's gonna make a mess inside your Mac as it wears the fan down, and may damage the motor eventually.
Then again, it may go away by itself, once it's worn the tips of the fan off!