Galacnor, Williams machines have a Hold Coil that frequently hums, it activates when the left flipper button is pressed and consists of switches that pre-set the machine so it is ready for play.

With your background all that is needed is a little study;

Learn what normally open or normally closed switch positions look like.

Put a bright light in your machine with the playfield raised and just look closely. It quickly becomes apparent. Relay switches skewed to one side bending all blades into an unnatural position. Cut wires. Rodent-eaten wires.

A cheap Mechanics Stethoscope can pinpoint exactly which relay/coil is humming. So can a paper towel holder against your ear.

Learn to set your machine to freeplay, this eliminates a lot of coin option problems.