There is an occasional click sound coming from my Macbook Pro (17", 2.2 Ghz Core 2 Duo) I upgraded to Snow Leopard recently, but the computer was also in for service about week before that, so I can't be certain the OS upgrade is related.

The sound has no set interval and frequency varies from rare to every few seconds.

I can alleviate the sound by turning up the fan speed with smcFanControl. Turning the right fan up about half way seems to be sufficient. Unfortunately, turning up the left fan also causes the right to turn on, because the sound seems to be slightly more to the left. The cause seems to be either a fan or heat-related.