TL;DR

A louder-than-normal hum from a transformer is often DC offset from harmonics or loose core laminations. Measure the supply voltage distortion and check whether neighboring equipment is injecting DC or harmonics onto the line.

What you might see

audible humming louder than the normal background level
humming changes in intensity with load cycles
hum is at 120 Hz rather than 60 Hz
nearby sensitive equipment experiences interference

Likely causes

Tap to mark as ruled out

DC bias from half-wave rectifier loads or asymmetric power electronics saturating the core
High harmonic distortion (total harmonic distortion above 5%) driving extra core vibration
Loose core laminations after years of vibration fatigue
Resonance between the transformer tank and the mounting pad at 120 Hz

Required tools

Power quality analyzer (THD and DC offset measurement)
Sound level meter (optional but useful for trending)
Arc-flash PPE and category IV probes for secondary measurements

Safety first

All measurements on the transformer secondary service bus must be performed by a qualified electrician with appropriate PPE. Secondary voltages (typically 480V or 208V) are lethal.
Do not open the primary or secondary connection compartments without utility isolation and a switching permit.

Procedure

6 steps · ~45 min

1
Take a baseline sound level measurement at 1 meter from the transformer and record it. Compare to historical readings if available.
2
Connect a power quality analyzer to the secondary service and measure total harmonic distortion (THD) and DC offset on all three phases.
Warning: Secondary service measurements require category-rated equipment and appropriate PPE. Use only a qualified electrician.
3
If DC offset exceeds 1% of peak voltage or THD exceeds 5%, identify the source: half-wave rectifiers, arc furnaces, or VFDs with regen units can inject DC or low-order harmonics.[1]
4
Install a DC blocking capacitor or a line reactor between the offending load and the transformer secondary bus if the source is identified and cannot be removed.
5
Check that the transformer is firmly bolted to its pad. Loose anchor bolts allow the tank to resonate at 120 Hz.
6
If harmonic content is within acceptable limits and the unit is still louder than typical, arrange a service inspection with ABB or a qualified transformer service firm. Loose core laminations require an internal inspection that must be done by a specialist.

Sources

ABB Distribution Transformer Operation and Maintenance Manual, ABB
ABB Pad-Mount Distribution Transformer installation and maintenance instructions, harmonic loading and sound level considerations (general)
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