The most common thought is that the harmonics results from the load connected to the transformer. However, I can agree that the iron lamination condition can contribute to excessive noise.
Loud sound can be relative. If you can somehow check the frequency of the sound and distinguish it from 60 cycle hum, you may find that it is indeed a result of nonlinear loads. Sounding louder when the load is on is a sign it may be caused by the nonlinear loads.
