I know the answer to this one because I did it to mine. I guess you can call it a mod. I would call it a defect! Apple put these crap fans in the computer, and when they reach high speeds it makes this subtle but extremely annoying sound that will drive anyone up the wall that is trying to concentrate. Here is what I did. I tried everything myself, I tried padding it around the fan, re-situating it, and more; nothing worked. So, I decided to take the other route. I hope this is still available to you. I called up apple, I was like look, this fan will nojt stop with the noise. So they made me carry the damn thing inside the mall to the appl store, up two stories on the escelator and all! I was really pissed, then I got there inside of the apple store and its lound in there, so the noise of course can't be heard! Well I get back home and the noise starts back up again. So, I call apple back up and I am like look I want t new fan, I am not stupid I know about computer and I know what the problem is and everything, the guy says that he can't do anything unless I send it somewhere else. So, I ask to speak with the level 2 technician. I am blunt with the guy tell him what the deal is and he says, ok whats you address, I will send you a new one. So he sends me a new one, I install it, and nothing, silence! It was perfect, the fan is crap, get apple to get you a new one. If they won't, then buy one, they can't be more than 10 dollars. Anyway, I know whats your problem, it was mine.
"Fix it 'til it Breaks."

Jacob - EiC & Director of Technology
Mac Pro Quad 2.66 - 4GB RAM
160 GB SATA RAID 1 - 650 GB Storage
Quad 19" Widescreen LCDs
Accessorized to the Hilt