Clicking sound on MacBook Pro

I had this problem and managed to solve it without any unscrewing, purchasing, or taking to the genius bar.

Firstly, if you have an old computer with a hard drive, this is a terrible idea, but if you insist on doing it anyway, power off your computer and some time to allow the disk to slow to a halt. This may not be a permanent solution, but before you go unscrewing your computer and voiding your warranty, I recommend flipping it over and give it some gentle hits (like you would knock on a door, I guess, unless you're someone who knocks really hard) along the side where the screen and body connect, if the sound might be coming from there. It's where the fans are and they're held magnetically, I believe, so it probably just got dislodged and was clicking against something. After I did this, the fan made some slightly worrying noises before going silent (wasn't doing anything intensive), but after running a high-intensity app (KSP), the fans whirred up like they're supposed to and didn't make any clicking sound.

Common symptoms of a dying hard drive

Hard drive is making a clicking, whirring, grinding, ping pong ball noise or any other type of unusual noises.

Hard drive is not spinning.

Computer is not recognizing the hard drive.

Sluggish loading of applications.

System freezes and hangs.

An interesting site - Hard Drive Sounds by drive manufacturer.

Backup your data immediately, as your hard drive could be failing. Then run the Apple Hardware Test:

http://support.apple.com/kb/HT1509