If it is not covered by any warranty anymore-

You can look up how to clean the fan & internal parts, it isn't hard to do, but easier if you look up How to do it online- than if I try to type it out..
there are videos on youtube of how to do it too.

If you have pets or even carpet near the computer- the hair, lint & extra dust can wrap or stick to the inside parts.

If the noise is due to dust/dirt , lint it will quiet down after cleaning, and if not then it might be the hard drive making the noises. or the fan bearings going out.

Sometimes if you can listen carefully you can pinpoint what area it is coming from.
then open the case if desktop (de static yourself) and see what is in that location of the noise.

Or you can Google images on that brand/make/model to find internal images that might help you to figure out what might be the noise maker location, if you don't want to open anything up.