I made a brief review of some spec sheets for noise levels, many quote ISO 7779 and/or ISO 9296 testing. Here are the idle levels they supply:
Model Sound Power (Deskside)
Z800: 46 decibel (28 decibel)
Z440: 42 decibel (26 decibel)
Z240 34 (17 decibel)
Lenovo P510: 38.6 (30.6)
Fujitsu M740: 37 (21 decibel)
For the HP Z2 Mini workstation, the noise emissions are missing from the quickspecs. The heading is there, but the values are missing - can somebody from HP please fix that?