Some PSUs will (and not even when under pressure of use) emit a loud rapid flicking sound - its not the fan working overtime, but the alternator (or some other electical thingy that I have forgotten the name of) working overtime. It might not even happen when the PSU is being used intensivly and might be silent when it is - its one of those strange comptuer things.
Best thing is to give it a listen to and see if its too loud for you - I don't think it will ever damage your computer but it can be annoying if it starts happening all the time