I am working on an inexpensive System 11C power supply for LED displays only (no HV). I bought a set of LED displays for Dr Dude(who does not hum as it was) to test with and after installing them I immediately noticed a tad bit more hum related to the display switching on and off that did not exist with plasma displays. I tried my first version replacement (switching) power supply and it actually hummed worse than the stock board UNTIL i put 470uF cap across the 5v right next to the display power supply connector. That cap pretty much cleared up the display switching noise entirely. 470uF cap near the 5v pin of the display power plug also helped a stock WMS PS board.

Sort of off topic, and the issue did not crop up until LED displays where added, but a lot of DE's hardware is a bootleg of WMS. Something to consider trying, more 5v capacitor filtering near the source of what is making the noise.

More load switching on and off making noise, like what I see with LED displays. Normally the 12v/5v (not sure what color dmd uses) load is pretty constant. When you add something switching on and off seems increase likelihood of noise.