My PC used to make some noises before, too. If your PC is under your desk, try scooching it back a few inches. Could be that it's in a sensitive area. On a technical approach, though, you could follow Johnny's advice and turn the fan speed down. Turning it down will reduce the rate at which it is going, and will cause the computer not to vibrate as much, or make as much noise. You may also want to open up the CPU and use a compressed can of air to clean out the dust. Sometimes cleaning out the fins of it can help the rotations of the fan. Doing this in conjunction of reducing the fan's speed will have a double edged sword here:
Reducing fan speed may result in a quieter PC, but it will cause temperatures to increase. Your PC components are cooled off by these fans, and by them getting too hot can possibly cause damage especially to a PC that's 5 years old. You can enter the BIOs (where you can reduce the fan speed) by restarting your computer and looking at the on screen instructions (It will come up briefly) and hit the key corresponding to the "Press the (F1 - F12) key to enter setup". It is in these options you can configure hardware. Becareful, fiddling with options may result in damages that are irrepairable. Good luck!
the old mother board short circuited, everything in the computer is new, its just the case. and i think i found the causes. see the case is an alien ware, so it has plastic on top of the metal case, and apparently that has enough room to vibrate. so i took a old sock and have it pushed up against the left side panel and on the right side, if i keeps acting up, im going to add small foam incerts. this way it will dampen the vibrations, and i also have a 10lbs dumbbell on top of cloth on top of the case so allot fo the vibrations stopped there
i know the fan is blowing faster than it use to, because i never had air blow in my face and dry my eyes out before. and the inside cant get hotter than before i replaces the geforce 6800 ultra with a hd 5770, and the cpu use to be a prescott p4, now is a phenom II 955.
i'm ok with the noise the fans make, its the vibrations that make the case make noise that i hate.