What do you mean by "noisy"? I've never heard of a driver board make noise. If you are talking about the DMD itself, yes those do make a high pitched noise that changes pitch depending on how many dots are displayed. Your driver board is now producing the proper voltages so that's probably why you hear it more now. Some manufacturers of plasma displays just seem to make more noise than others (Vishay, Cherry, Babcock).

If you add the colorDMD, it's going to bypass the HV section of the board anyways. That was the reason your DMD probably wasn't working properly to begin with.

I understand that. It's your DMD causing that. The reason why it wasn't that loud before is because the driver board wasn't outputting the proper voltages. Any rebuilt or new driver board will cause that same display to make that noise now. Lowering the zener diode value (or a failing zener) will cause the sound to get quieter and the display to get dimmer to the point it can't illuminate (which is where you were).

I kind of like that 'singing" of the display. Probably best to get a colorDMD to eliminate it altogether.