If it's not your fan (which you say you've cleaned), then the only other thing that can make a noise is the hard-drive (HDD).
You shouldn't normally be able to hear it, but If it's making a frantic 'tock-tock-tock', sound then failure maybe imminent - back up now !
Once you have saved your data, you can attempt to run hardware tests - for instance, in XP there is an error-checking function: in My Computer, right-click on hard-drive icon, Properties, Tools. Error-Checking, select 'Check Now...' tick both options, and start. You will then have to reboot for it to run.
If this does not fix the problem, and you have managed to save your data, it may require a new drive - depending on the state of the drive, you may get away with cloning it to a new one, which can just be popped in.
Can you get access to the HDD? Some just pop out, others may mean some disassembly of the laptop to get at it.
Hope this is of some help.