My Williams System 11 "Fire!" had a terrible buzzing noise that would correspond with the light matrix. This was especially noticeable in attract mode with the sweeping/dancing light show. When the light matrix was off, it was almost completely quiet.
I tried swapping sound boards with a known good one. This did not fix it.
I tried powering the sound board with a separate PC power supply. This did not fix it.
I tried unscrewing sound board and letting it float free. This greatly reduced the lamp matrix buzz by about 90%!
So I went to Home Depot and bought some 8-32 nylon screws and nylon washers and now have the sound board screwed in with these. Apparently there was some kind of ground loop happening between the light matrix and the sound board.