When you power it up the first time, leave the pc running for an hour or two with all doors shut and then see if the air inside the case is warm. If it is, you could get a nice quiet 92mm fan and grille, and stick it in the back of the case somewhere.
I have a Panaflo 92mm fan on my processor heatsink, and I can't hear it spinning at all, even with all other fans off.