This noise you're hearing - is it a mechnical noise, or could it be something coming out of a loudspeaker or an electronic buzzer?
There is a small loudspeaker on the system board, nowadays only used to make the "beep" when the computer is switched on, but in the days before external speakers programs also used it to play musical sounds.
On the other hand, the most likely mechanical parts to be making a noise are:
The fans. The case will have at least one fan, maybe two. The video board may also have its own small fan, unlikely to make much noise. If you think it's a fan, carefully clean out the dust and then oil the bearings as suggested beforehand.
The hard disk. If that is making a loud noise, then it's on the point of failing. If you've got anything stored on it that you care about, back it up immediately if you haven't already done so.
The CD or DVD drives. They are only likely to make a noise when a disk is in them and in use for reading or writing.
Try to figure out which of these is the source of the noise and let us know, and people here might be able to help.
Alternatively, if you're not experienced with working inside a computer then after backing up your data it would be best to put it in for servicing. You can safely assume that whoever services it may well delete everything on it, especially if they have to fit a new hard disk.
If you decide to work inside the computer yourself, then disconnect it from the electricity supply and the phone line, and use a wriststrap to prevent static electricity discharge. If you don't understand that advice about static electricity then I'd say - don't work inside it.
Good luck, here's hoping you can get it fixed,
John