Turn the PC off and unplug everything. Sit it on a level surface and gently but not too gently rock it from side to side in an effort to dislodge anything that may have fallen into the way of fans or what not. Plug it back together and see if it goes back to working the way it should. My desktop at home did the same after playing with its guts and this solved it as it appeared to be some dust near the fan. If it doesnt go away then you may have to open the case up and see what may be causing the noise.