Hum in a transformer is caused by eddy currents developing in the iron plates inside the unit. Other than being annoying, they don't cause a safety issue of any kind. However, that energy is part of what keeps a transformer from approaching 100% efficiency.
An eddy current is caused when a magnetic field crosses something that will conduct electricity. The field then induces a current in the conductor. In this case, the current is induced into the metal plates and the plates are vibrating due the fact they are in a transformer where there is a large magnetic field. It is a form of a speaker in a way; it is the same principal.
If this is an inexpensive isolation transformer, then some hum is probably expected. If this is an expensive unit or one made for audiophile systems, I would either return it to the dealer or have the manufacturer repair or replace it. There is no excuse for noisy annoying hum in a good quality isolation transformer. The more hum, the less efficient and the lower quality of design and manufacturer.
An eddy current is caused when a magnetic field crosses something that will conduct electricity. The field then induces a current in the conductor. In this case, the current is induced into the metal plates and the plates are vibrating due the fact they are in a transformer where there is a large magnetic field. It is a form of a speaker in a way; it is the same principal.
If this is an inexpensive isolation transformer, then some hum is probably expected. If this is an expensive unit or one made for audiophile systems, I would either return it to the dealer or have the manufacturer repair or replace it. There is no excuse for noisy annoying hum in a good quality isolation transformer. The more hum, the less efficient and the lower quality of design and manufacturer.