Install different fans?
Simply putting in a lower rpm fan would probably reduce the noise, but
you don't really want to do that, as presumably Dell had a reason for
choosing the higher speed fans. Same potential issue with whatever it
is you did to the BIOS.
Check what fans you have, maybe they're cheap sleave bearing models.
Installing an otherwise identical model with ball bearings will
probably make it quieter. If that's not enough, you can try and get
fancy and use variable speed temperature sensing fans, or something
like that.
What are they, 80 mm fans on the back of the case? If so, there's
some electronic catalog that usually has a nice deal on Panaflo 80 mm
fans, something like 10 for $20, while most computer sites will try to
charge you $10 just for one fan. I can't seem to remember the name of
the catalog, though...
