Other than your PC speakers, noise in a computer comes from 2 places fans and drives.
Sometimes its a worn out fan or a wire or something laying on a fan. A bad barring may make noise sometimes but not always.
A bad drive or a disk in a floppy or CD drive can also make noise. If the noise is coming from a bad hard drive then it could make the computer run slow because the drive is working harder.
Make sure your floppy and cd drives are empty.
Using a plastic pen cap you can test to see if its a noisy fan by stopping the fans one at a time.
Normally you can stop the power supply fan from the back of the computer by poking the pointy end of the pen cap in the fan hole on the back of the computer. If the extra noise stops you found it. You will know you did it right if it sounds like you just stuck a pen cap into a fan blade.
To get to the CPU fan if your computer has one. Open the case and look for one or more fans and do the same thing you did with the power supply fan.
With the case open you can try to figure out where the noise is coming from. If all the fans are stopped using the pen caps the only noise would be drives.