I have that rheobus from SVC. It doesn't work very well with Panaflo fans due to how it varies the voltage: pulse width modulation. They sounded awful when turned down. It works ok with my Sunons. Apparently some fans don't like to be turned down, at least with this method. I tried soldering in my own rheostat inline with one of my panaflos and it didn't make the awful noise. The rheobus itself IS silent, even at minimum voltage.