I've found the worst hacks in games owned by engineers, technicians, and electricians. They all think they "know better" than the (very qualified) mechanical engineers the pinball companies employed. Hack city!

The chemical engineer who clearcoated 2 EBD playfields which both looked very nice - however, he neglected to remove ANYTHING from the top of the PF, so there was lots of curling up clearcoat next to every rail and post. Great job, fool.... you win one internets

There is a low voltage switch stacked with the EOS switch which should have a plastic interrupter/lifter on it. This is on the right flipper - they hadn't started putting lane change on both sides yet.

If you go into switch test you can flip the right flipper and hold it up you should see the # for the lane change switch in the display. If you don't something is amiss.

Whatever you do, *DO NOT* adjust anything with the flipper end of stroke and lane change switch with the power on - it is EXTREMELY EASY to slip and short solenoid voltage to the low voltage switch, since they are on the same stack!

Lane change switch is GRN-BLU (col) and WHT-BLU (ret) - perhaps someone disconnected it and it's taped up and tucked away somewhere?

There should be a diode there, too, looks like it's missing. You need to put the wire back on the lug it was on and add the diode.... refer to other switches in the column to get the orientation correct.

Have you ever tried isolating the sound board from the brackets it's attached to in case it's a ground loop?
(old trick from car audio days)

Isolate it from the bracket - people have used plastic washers and screws to do this, or plastic standoffs. The ground is still solid through your connector. The goal is to eliminate a differential in resistance between the connector ground and the metal bracket/backplane - in theory, they should always be the same, but in practice, small differences could contribute to hum.