I agree with Mac29, that definitely sounds like a fan.
If it is a desktop you have it is not a major operation. Take the side off with the computer shut down then see if any of the fans 'waggle' if you play with them (gently!). You probably have at least three fans - one on the CPU; one on the case; and one inside the PSU. There may be more depending on the spec of your computer and case. CPU and case fans are fairly easy to replace, and usually fairly cheap. If it turns out to be the PSU fan you are probably better replacing the PSU. PSU fans are replaceable but require opening up the PSU. This is a job that requires considerable care as potentially lethal voltages can be stored in PSUs for a long time after they are switched off and unplugged. Better to replace.
There is an old trick to better locate the source of the noise. Take the side off the case and start the computer running. Then take a reasonably long screwdriver with a solid handle. Place the tip near each fan in turn and the handle against the side of your head. It does work, honest !