I have only dissected one Dell. It was a discard, Slot 1 440BX mid tower. The cpu exhaust fan was loud, but that could have the result of years of operation. It had ducting to the fan which was seperate from the powersupply.
I use a Dell Optiplex 2.4c P4 at work. It is dead silent. Since I did not choose it or can complain about it not working, it is okay by me.
At home, I have a Lian Li case with an XP processor and a SCSI 10K hard drive. When everything is quiet, the hard drive will make you think an ice pick is being pushed into your brain. As soon as anything else makes noise, it disappears. My hearing is far less than perfect. Sounds are very subjective. Dells are quiet and most desktop versions are not housing SCSI hard drives.
I think I could turn off some of the fans in the Lian Li, or at least turn them down a notch.
Lian Li cases, out of the box are not very quiet. I am still looking for some suitable sound dampening material that is easy to apply to the large flat surfaces inside the sides, bottom and top.