I have the Pioneer 420. Mine makes a slight scraping noise when the HD is recording (and possibly playing, but I don't notice it over the sound of what I'm watching). If the room is very quiet, the sound is audible and a bit annoying. Other than that, very quiet, like when I'm burning to disc.
P.S. Strange -- the word Pioneer automatically turns into some kind of link.
I also have a Pioneer 420. When I am burning a disk, to date always copying from the HD, the unit doesn't appear to be making any greater amount of noise than any other component I am running. I'm usually watching the tube while burning vs. listening to classical music, that might be more adversely harmed by ANY background noises.
PS - strange that "running" became hyperlinked to a non-relevant site .... hmmmmm
RCA DRC8005N. It is extremely quiet. I haven't really noticed it making any sounds other than the loading and unloading. If the fan has come on, I haven't been able to detect it. If it is very quiet in the room, one can hear a slight clicking sound when the unit is "finalizing" a disc.
It is in an entertainment center; but the Akai DVD player that previously occupied the same space was very loud. Obnoxiously so.