Forum OpenACS Q&A: Re: Dell Poweredge2400 Fan Noise
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...