[DamnthatDavid]

You should have another fan besides the processor fan. 2-3 or so.

That sound, is the sound of the fan slightly loose. My computer had that problem, so I took off it's case, and poked the fans while they where still on. Then I slipped and busted a fan blade off, which sent the entire thing into a crazy wobbly vibration. Long story short, I had my computer's side opened for half a year with a desk fan blowing into the case.

Just tighten the screws a bit, and that normally fixes up the problem.