What kind of humming is typically associated with transformers under certain conditions?

The type of humming that is typically associated with transformers under certain conditions is known as transformer humming. This phenomenon generally occurs when the transformer is energized, especially when a switch is closed, causing the core of the transformer to vibrate due to alternating magnetic fields. This vibration generates an audible humming sound that can sometimes be perceived at various distances from the transformer.

When a transformer is energized, the transformer's core undergoes magnetostriction, which is the change in dimension of magnetic materials when they are magnetized. This effect leads to mechanical vibrations, producing the characteristic humming noise. The sound intensity can vary based on load conditions, design factors, and even the frequency of the electrical supply.

The validity of transformer humming being associated with a closed switch specifically highlights the connection between switching operations and the generation of sounds, making this the correct answer in relation to the question posed.