A lot depends on the condition of the platen and the particular adjustment of the machine, but a Royal Quiet De Luxe can be very discreet. Yesterday I tried an early-’60s Skyriter that was surprisingly quiet. The “noiseless” portables avoid sharp taps, but they are hardly silent.
Clackety Clink 
A lot depends on the condition of the platen and the particular adjustment of the machine, but a Royal Quiet De Luxe can be very discreet. Yesterday I tried an early-’60s Skyriter that was surprisingly quiet. The “noiseless” portables avoid sharp taps, but they are hardly silent.
Late 40’s Royal QDL, and most 50’s era Hermes Rocket/Babys. These are the quietest I own, and platen hardening doesn’t seem to make them much louder.
I’ve been trying to avoid acquiring more machines until I sell a couple of my existing ones, but an ultraportable Hermes DOES sound appealing.