Well I have a grinding noise on the inside of my Mac Pro somewhere. I thought it was the forward most chassis fan. It turned out to be the fan that is built-in to the heat sink of the CPU.
So when the Apple Store replaced the fan I originally asked them to replace the machine came back scratched. It is not that bad and more of an annoyance then anything. I'm so neurotic about keeping my toys meticulous.

Sometimes I think it gets in the way of enjoying them... anyways
I am not really mad at the tech's at the Apple Store either, I spend so much money there for business they basically take care of any problems I have with no questions.