A relay buzzes while energised.

EM relay buzzes or hums loudly

Cause: Loose mounting, an armature not seating, dirt, a worn half-moon dent in the armature where it meets the core, or too much spring tension.

Checks

• Check the relay mounting screws.
• Check the armature seats and its spring is hooked correctly.
• Look at the armature face for a crescent-shaped dent.

Fix

• Tighten or replace missing screws.
• Clean out debris.
• Grind the dent smooth or replace the armature.
• Try a slightly longer or weaker spring.
• As a last resort, damp the noise with a thin layer of electrical tape or a sticky label on the armature face that strikes the core.

Systems

• Electro-mechanical

Related tips

• A hold relay hums or buzzes
• Williams relay hum from a missing armature rivet
• Gottlieb relay return springs
• How latch-trip (interlock) relays work
• Relay banks and their reset coil
• Relay coil resistance and frame sizes
• Gottlieb AS relay (miniature match stepper)
• Push relays in by hand gently and straight

Source: Pinball Machine Care & Maintenance, Bernard B. Kamoroff (Bell Springs Pinball Alley)