|Its not recommened you open a PSU unless you are qualified. The capacitors hold a hell of a lot of charge and it could be dangerous.|
take out the PSU and while it is OFF and unplugged try to turn the fan with a pencil. see if it is in fact touching something, if it is a loose fan well then you have two options, either tighten it (yourself if you want to or take it to a shop) or buy a new PSU.
if it is not the fan, then as OtheHill suggested abpve, it could be a twisting of the case/flimsy case etc which to me sounds like to be a higher possibility.