I used this thread a lot to trouble shoot all the noise my Data East Jurassic Park was making, so I figured I would let everyone know what worked. When I brought it home several months ago, the humming noise was loud. I tracked it down to the DMD (by unplugging the DMD, the noise would drop) and to the light show in attract mode. I got rid of the noise by buying a new DMD and by isolating the sound board from ground (used electrical tape around the screw holes). This got everything quiet.
However, months later I started to have some flaky power supply things happening so I bought a new Rotten Dog power supply board. When I installed it the humming and buzzing was horrible. You could hear it all over the house. All of it was coming out of the speakers all of the time. I recapped the sound board and that knocked it down to a more tolerable level, but it was still louder than I wanted.
So I bought the power supply mentioned in post 442 (Mean Well RT-50B) and a connector. Once installed this dropped the noise down to zero. It is now perfectly quiet. The power supply was easy to install as I just mounted it in the cabinet near the speaker and spliced into the 120V going to the coin door (which isn't even used unless you have a bill changer). I ran 4 wires up the the sound board, hooked the wires to the new connector (you need a special tool for this), and plugged it in. If someone wanted to remove it (although I have no idea why you would), you could plug the old connector back into the sound board and splice the 120V cable back together.