Hi jomuir. When you say that the fan is noisy, are you referring to the sound of air turbulance or to bearing noise?
A computer generally has 3 fans, sometimes 4. There is a fan in the PSU and one on the processor heatsink Modern graphics cards generally have a cooling fan and some motherboards have a fan on one of the chips.
The most common fans to develop bearing noise are the processor fan and motherboard fan. The easiest way to find which fan is making the noise is to take the side cover off the computer, then roll a sheet of A4 paper into a long tube and with one end against your ear use it to locate the noise source. The best way to deal with the problem depends on which fan is making the noise and whether it's air turbulance or bearing noise.