TL;DR

A hum or buzz from the panel is usually a loose bus bar connection or a breaker under high harmonic load. Locate the noisy component with a listening scope and re-torque the bus connections.

What you might see

audible buzzing or humming from inside the panel
noise increases with load
one breaker noticeably louder than others
panel cover vibrates slightly

Likely causes

Tap to mark as ruled out

Loose bus bar clip connection on one or more breakers
Breaker body resonating under high harmonic current from VFDs or switching power supplies
Neutral bar looseness causing the bar to vibrate at 60 Hz
Foreign object or debris touching a live busbar

Required tools

Torque screwdriver
Insulated screwdrivers
Arc-flash PPE
LOTO kit

Safety first

Never touch the main breaker line-side lugs with the utility connected.
Wear arc-flash PPE rated for the panel's incident energy whenever the dead-front cover is off.

Procedure

8 steps · ~30 min

1
Listen carefully to locate which side of the panel the noise originates from. Note whether it changes under load.
2
Open the main breaker and lock out the upstream feeder.
Warning: The main breaker line-side lugs remain live. Keep hands clear of the top of the panel.
3
Remove the dead-front cover.
4
Press on each breaker firmly to confirm it is fully seated on the bus stab. A loose stab will have visible rocking.[1]
5
Re-seat any loose breaker by pressing firmly until the stab fully engages the bus bar clip.
6
Check all neutral bar and ground bar screws. Torque any loose screws to spec.
7
Inspect for loose panel knockouts, screws, or debris that could vibrate against the bus.
8
Restore power and listen again. If noise persists after re-seating, check the load circuit for high harmonic content (non-linear loads like VFDs or UPS systems). A line filter on the offending circuit can reduce the harmonic current causing the resonance.

Sources

Square D QO and I-Line Load Center Installation Instructions, Square D (Schneider Electric)
Square D QO Load Center installation instructions, breaker seating and bus bar inspection (general)
View source

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