I have owned three generations of MacBook Pro Unibody and all three generates a subtle but annoying clicking noise after a long use and increases frequency as I click the touchpad or typing with it on my lap. Luckily I am not the only one with such problem, and if you own a recent 15″ model MacBook Pro there’s a good chance you are experiencing it too. Here’s a YouTube recording of the noise.
Ten One Blog discovered the source of the noise (no, it’s not the sudden motion sensor) and found a permanent fix:
One of the connectors in this system touches the bottom plate right in the center where it flexes a lot. When the computer heats up, the metal pins in the connector scrape noisily against the cover and chassis. It makes you want to eat your hands.
By removing the connector (two easy screws!), you remove one of the grounding connections between the unibody enclosure and the bottom cover.
I chose not to remove the connector but to stick an electrical tape on the back of the bottom case where the connector touches, which seems to have fixed the problem.
Jump to the linked post for some pictures and more details.