The noised may be caused by one of the components of the computer or devices attached to it. You should check them out closely to see which one is the cause. It might also be due to power supply. The power supply is also a common hardware component that can cause an excessive amount of noise.
The fanblade assembly may have moved slightly on the shaft of the cooling fan, causing it to make contact with the shroud or the protective grille. It's usually mounted at the rear of the cabinet. Find the grille where the air is blowing out and try to determine if that's where the noise is coming from. If it is, the fanblade assembly can usually be carefully moved on the shaft to gain sufficient clearance to stop the noise. AnneKay's right about the other noises, but this is the only one I would characterize as being "really loud".