Hello,
There are three sources of noise: hard drive, disc drive, and fan. IMO, hard drive noise is a non-issue as they are very quiet. Disc drive noise is very high on all three machines, but I'm assuming you wouldn't be using the disc drive as a real-time playback device. What one really needs to be concerned about is fan noise. That said, I do not recommend tweaking fan settings (temperature, rpm, etc.) as that can effectively reduce component life. In my experience, the iMac has its fan turn on the least (if at all) during normal operation compared to both the Mini and the MacBook, probably due to its larger form factor. I use an iMac in my music room at about 10' from the listening position and cannot hear it at all. The smaller machines tend to turn their fans on much more frequently, but this may be a non-issue if they are placed where there is adequate ventilation and are only used for streaming.