I've been using Respironics SystemOne for five years and in that time had two different machines and three different power bricks (always the same models, just exchanged because my supplier changed and one power brick failed).
All three power bricks produced a high-pitch noise, a noise that is familiar from cheap battery chargers or cheap power supplies, but nothing I would expect from medical equipment. They don't always do this, and the sound is not very loud, but it varies and the high pitch is escpecially bothersome trying to find sleep or read a book before sleep (it also occurs if the machine is not active, even if the power supply is not connected to the machine).
I always put this down to bad luck with the bricks I got, but just having gotten the third in a row with that problem it seems like it's a general problem, so I thought I'd ask.
Picture of the brick attached, not sure which of these numbers is something like a model number ;-)