A relay that stays energized makes a loud 60 Hz buzz.

A hold relay hums or buzzes

Cause: Too much spring or switch-blade pressure. On AC the coil's pull drops to zero 120 times a second; if the armature is fighting a stiff spring it lets go slightly at each zero and snaps back, making the noise. A coil with shorted turns is also weaker.

Checks

• Measure the coil: it should be 30 to 150 ohms.
• If another relay in the game uses that coil number, measure it too; both should read alike.

Fix

• Replace the coil if it reads low.
• Otherwise set the switch blades and return spring for the least tension that still pulls in and returns the armature; a longer, softer spring helps.

Systems

• Electro-mechanical

Related tips

• EM relay buzzes or hums loudly
• Quieting a buzzing coin lockout relay
• Gottlieb hold relay runs the whole time the game is on
• Williams relay hum from a missing armature rivet
• Game resets but will not start: adjust hold relay and reset-completed relay latch
• Gottlieb relay return springs
• How latch-trip (interlock) relays work
• Relay banks and their reset coil

Source: Repairing Electro-Mechanical (EM) Coin Operated Games to 1978, Part Two, Clay Harrell (pinrepair.com)