Generally, the only two moving parts in the computer are the fan and the hard drive.

If the noise is constant, its the fan. Or a fan, since modern computers tend to have multiples. Open up and uplug each fan in turn until you identify it.

If it starts and stops, its the hard drive. Can you say 'worst case scenariao'?

The third possibily is that the case is loose. Place your hands on the case. If the sounds quitens down, then tighten the screws holding the case in place.