isolation transformers hum

my giant Noisetrapper 2000 isolation transformer hums audibly, and the hum gets louder as the load on it increases (e.g., turn on tube amp in standby, it hums; switch it on all the way, gets louder). Do all isolation transformers hum? Could it be caused by dc on the line, that could be filtered out by one of those ps audion dc filter devices?
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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.
Not familiar with that brand and couldn't find info.

Most of the better brands are rated for below 50dB, under 5KVA. I've got two (5000VA and 500VA) and can't hear either without a stethoscope. A third should be soon.
Hasn't always been the case and had one that was always noisey but I got it for free, so it didn't hurt to abandon. The big one has made noise when wired one way, but only after I moved to another house. Unpredictable.

Saturation and noise can be caused by DC offset and the PS Audio Humbuster can reduce that but it's only one of many factors.