The loudest fan (finally died)
The left fan in my 2006 Macbook Pro had been loud for a long time. Finally, under what I assume was extreme duress, it got REALLY loud and cantankerous, before totally stopping dead.
I ordered the replacement fan, used the iFixit install guide, and Shazaam, it was good as new, and much, much quieter.
BUT, a problem arose: A day or so later, I was (legally) ripping a DVD onto my computer, and the whole computer died. Uh oh. It came back on fine, and I tried it again. Same thing. The ripping was somehow causing the whole computer to shut down.
THEN, I realized the computer's case wasn't all-the-way snapped shut. I haven't had a problem since. (Granted, I also haven't tried to rip a DVD, so the jury may still be out.)
Use the opportunity to clean out some dust around the fan area. I had a good chunk of it in there, and I used compressed air to blow it away.