come t think of it it does sound like electric but not entirlykillakrust said:Crackling like 'Electricity' is a bad thing, and should be checked.
If it is just an annoying rattle (many PC's have them) then there are various ways to proceed.
1. Check all screws are tight on the case (not too tight)
2. Check the side panels are correctly mounted.
3. Try and locate the source of noise. Is it a worn out fan or failing drive?
Sometimes the source is a case that is too flexible. Remounting the cd\dvd drive or hard drives in a another slot can sometime stiffen the chassi.
Last ditch effort is to replace any rattling hardware with higher quality or new parts.