umm. if its a bottom intake psu (psu at the bottom of case pulling it from the bottom) isnt it going to be sucking the warm air back in?
hot air rises, would mean the fan would have to work harder to push the hot air away, meaning faster fan speed = bigger noise
unless u could mount it sideways to push the air out the side.
use a stiff piece of cardboard cut into shape, a cereal box or similar and a bit of tape ...job done and cheap as chips.
What psu is it? I would be more concerned that you feel its exhausing hot air, it should be warm at best.
If your psu is cooking itself, then fixing the symptom rather than the case is a little counter productive...