I'm almost sure this has to do with one of my 2 Western Digital hard drives. It happens periodically and I'm almost sure its a sound coming from a drive. I'll hear a sound from my case like something is powering down, then a second later power up again, like a fan cutting on. A couple of seconds after that the whole system locks up. If I simply reset then I usually hear the same sound of something cutting off and back on again, followed by a lock up somewhere in the boot process.
So I push on both IDE cables and power connectors of the hard drives really tightly to make sure they are getting connection. This usually solves the problem temporarily, but it happens again at random in any amount of time from a day to a couple of weeks. Was just curious as to what kind of problem this sounds like, be it the hard drives or bad cables.