So shortly after I uploaded the noise my MacBook Pro was making it got even worse. I pretty much took the time to sample the noise and then shut the whole computer down, and then pulled the plug and the battery, leaving it until such time as I could take it apart and confirm which fan needed to be replaced.
Such time happened to be this evening. So I took the computer apart, and fired it up again with the keyboard off so I could figure out which fan was making the noise.
Only neither of them were making noise like that. They were both working just fine. I even started up a fan control program and spun them up to 6000 RPM, and they still sounded just fine. I took the opportunity to vacuum out the innards of the computer, giving the fans extra attention just in case, and I removed all the keycaps and extracted a couple years’ worth of cat hair (this is not the first time I’ve done that, but I spent more time on it today, peeling apart the layers of the membrane to get as much hair out as possible). I also took more care in reassembling the keyboard, so all the keys seem to be sitting a little more evenly and there are no loose corners. The left cmd-key is a little stiff for some reason, but I’m sure that’ll be back to normal soon enough.
I still can’t figure out what was going on with the fan, but everything seems fine now. The fans are quieter than they were before the disassembly, so I guess the vacuuming had some positive effect. I’m a little disturbed that I didn’t really do anything and the problem still went away, though.
