There aren't too many moving parts, so you will need perhaps to lift the lid and get a better idea where the noise is coming from.
If it is a fan (CPU or graphics card or power supply), it might be fouling against something (sometimes there are plastic air guides, particular on CPU fans which can come loose). Or the fan bearings may be failing, and it will need replacing.
Alternatively it could be a drive (DVD or hard drive, or even floppy on the more venerable machines). If is a hard drive, make sure you have a backup before it fails completely.
Last edited by neonsignal; 02-19-2011 at 10:22 PM.