Most stuff in a metal box with screws in tend to be quiet and
commercial systems like DEC, SUN and HPs in full dress
tend to be quiet if made to the specs in effect after 1980ish.
PCs however are all over the map, some excellent and
some very poor.
But I know in the case of DEC that if they are open like my test beds are
then you can forget everything except REAL strong FM broadcast in the same
room. The 2M rig with an outside antenna even gets hit. This is in a metal
sided and roofed office trailer.
Dan