I have the same problem with my tmnt. I've unplugged the DMD with no change. It does seem to be voltage related though. When in diagnostics and running bulbs checks, the more bulbs on the louder the noise.

I have an update. I have replaced about 90% of my tmnt bulbs to LEDs. I didn't not see a change until I swapped out the lights in the splinter captive ball. After doing that the noise is almost completely gone. Its now just barely heard. The transformer is actually the only noise you can hear.

So I wonder since these bulbs are located between the speakers, could they cause interference when powered? Or could the low draw of LEDs fix this issue or did I do something else by accident?

Noise returns if I replace with old bulbs. (Only replaced ones around speakers) I do have slight hiss from speakers when the LEDs are installed. I'll deal with the hiss over high and low constant noise with the old bulbs. Its only a band aide on the issue right now but its a start.

I used coin takers LEDs not sure if they are polarized or not