CPU usage spikes


I'm running 10.4.1 on a 12" powerbook, 1.5 ghz. Recently I noticed the fan coming on more often than it used to, and I started watching the Activity Monitor's CPU history. I notice that usage tends to spike up from its idle every few moments (maybe around once per minute, give or take a few seconds). The graph jumps up to about 4 times its idle height, and remains that way for 5-10 seconds or so.

All I run in the background is Temperature Monitor and Quicksilver. After quitting both of them, the spikes still happened. Looking at the process list, sorted by CPU usage, no process is obviously causing the spikes.

It's not RAM paging or anything, I have 768 mb and plenty of room to spare there.

Is this common? I was thinking it could have something to do with spotlight indexing or something, but how often would that really have to run?