For some time, my 4+ year old MacBook (late 2009 model) has had an issue with the fan spinning up and being really noisy. For awhile, I thought it was just newer software (like the current GarageBand) being more CPU intensive, but Activity Monitor seldom showed anything agressing going on.
I used things like smcFanControl to show the temperature and RMPs of the fan, and only last night did I do some further searching and realize my fan was running much faster, and the temperature was much higher, than it should be.
I did some searching and found various people reporting the same issue, and also some claims that Apple started using smarter hard drive firmware that the Macs could talk to and detect temperature. I had swapped out my hard drive twice, and wondered if I was just running an incompatible hard drive that was confusing the MacBook…
…turn out, it was far simpler. I ran across this post:
…and he suggested just cleaning the fan. I took my MacBook apart (eight screws on the bottom, then one micro tiny fan plug and another larger plug – be careful with the small one – VERY careful, tiny pins), and then took three screws out to release the fan and one more tiny screw to open it up. I used a can of compressed air from Radio Shack (which I bet I’ve had for 15 years) and cleaned it all out. The rear air vent (inside) was also clogged.
After reassembly, the MacBook is quiet like it used to be.