The little cooling fans inside of Mac notebooks run upwards of 6000 RPM, and they can eventually wear out. People report symptoms of rattling and even loud grinding. Well, the fan MAY need replacement. Or perhaps it just needs to be relubricated.
When my 2007 MacBook Pro's left cooling fan started rattling really loudly last night, I thought I was in for a time-consuming repair. Every place I called had a 2 or 3 day lead time on the part and a 2-day turn-around for the repair once the part was in. The fan itself costs at least $40, and the labor charges were at least $60. I was irritated at the prospect of spending at least $100 on a 4-year-old notebook that I plan to replace in like two months when the new model comes out.
One of the places I called suggested that blowing the dust out might help. Dust can accumulate on the blades and throw it out of balance. I opened up the computer (according to an article like this) and aimed my can of 1,1,1,2-Tetrafluoroethane at it. That didn't work for me, unfortunately.
After some research and tinkering, I learned that some of these brushless fan motors can lose their lubrication over time, and moreover, they can be RE-lubricated.
This is what the fan looks like when removed from the computer. I wish I had taken a photo of the fan disassembled. You can unscrew the two tiny phillips head screws on the fan and lift the top plate and motor from the housing below. Then you can gently pry the black plastic circular hub (with the blades) away from the motor, and it'll pop off. What's left on the top plate looks vaguely like this. The circular hub contains a magnetic ring and a spindle pin that kinds looks like what's on the right side of this image.
I dipped a swab in some motor oil (sewing machine oil would probably be better, and soybean or canola oil might be adequate) and put a light coat on the metal spindle pin. I popped it back together and tested it. Now it's silent again, almost like new!
This may only be a temporary fix. The pin looked a little worn, probably because it had been rattling not too badly for months now. Was it noisy because the lubricant had migrated way? Or had it worn out? I don't know, but this isn't a brand new fan, so it's probably not going to last another 4 years before it needs to be lubed again or replaced entirely (assuming this notebook is in use for any purpose at that time). But it saved me $100, and I can now use my old MBP peacefully until I'm ready to get a new one.
[crarko adds: Obviously if you still have a warranty or AppleCare in place you should bring this to an Apple Authorized repair center to get it fixed. But, once out of warranty any repairs to the hardware can get quite expensive, so it's nice to have do-it-yourself options available.]
Mac OS X Hints