If a transformer "hums" after it is energized, what does it indicate?

When a transformer produces a humming sound after being energized, it typically indicates that it is operating normally. This humming noise is primarily generated by vibrations in the core and windings of the transformer, which occur as a result of the alternating current (AC) passing through it. As the electrical energy flows, the magnetic fields around the core expand and contract, causing the physical components to vibrate and create sound.

This is a common characteristic of transformers, especially at higher power levels, and does not necessarily signify any malfunction. However, if the humming is excessively loud or changes in pitch, it could be a sign of other issues, but under standard conditions, a humming sound is not a cause for concern. Other options pose scenarios that might involve maintenance or reliability concerns, but a normal humming sound is expected during operation.