Might be a Fan inside your computer, put your PC up on the desk and try isolate where the noise is coming from, maybe a blade of the fan is slightly bent and rubbing off the casing, or possibly a bearing... very easy to replace or try bend back into position... Noise might also be coming from your power supply or even your computers internal Speaker...
First give us an indication of where you think the sound might be coming from.
Remember that before you go poking around inside the computer you should have the power off and Earth yourself so not to blow anything from any static build up you might have.