If you tried different boards, then the problems is likely in your cabinet. Try another cab.
In your cab, check that the +12 isn't way out of wack (like +8 or +15 or something) +11 or +13 is fine.
Then disconnect the marquee light (even if it's not working) to make sure the ballast isn't causing the problem.
Then unplug the coin door, as sometimes feedback from the coin lights causes a hum.
But - I've found hums more prevalent when powering the board with a PC-style power supply, and virtually non-existent with a switcher...