The cabinet speaker hums steadily while the game sits idle; game sounds themselves are fine. On some games the pitch of the hum changes as the attract mode cycles.

Gottlieb System 3: constant hum from the speaker at idle

Cause: Ground differences: the cabinet speaker has no dedicated signal return (it returns through the transformer centre tap), and the sound board's mounting-screw ground and its separate regulator return can sit at slightly different levels. The speaker itself is not the cause.

Checks

• Check and reseat the sound board and power connectors, looking for loose or burnt pins.
• Listen with the sound board mounting screw backed off a little to see if the hum drops.

Fix

• Quick fix: loosen the sound board mounting screw and ease the board slightly off the backbox ground.
• Better fix: run a dedicated return wire for the cabinet speaker instead of relying on the transformer centre tap, and repair any poor connector grounds.

Systems

• Gottlieb System 3

Related tips

• Gottlieb System 3: loud screeching from the speakers: replace the sound board capacitors
• Sound barely audible though music plays: rebuild the aux power supply connector
• Where the boards sit in a System 3 dot matrix backbox
• Constant hum or odd noise from the speakers

Source: Pinball Repair help group!! (Facebook group)