Ubuntu :: Computer Unresponsive For 20 Seconds Every Day

Almost every day, for around 15-20 seconds, my computer becomes unresponsive, while the hard disk makes a strange periodic sound (period ~2 seconds, half of it the sound suggests very intense disk activity, the other half - normal disk activity), and the hard disk LED stays on (not flashing, just on). I ran manually all scripts in the /etc/cron.* folders. Only the mlocate script seemed to cause higher than normal disk activity (but still low enough to not cause problems). I still disabled it, but the problem persists.