Audio Noise from my step up/down transformer

P = I X E

P = power, watts, va
I = amps
E = voltage

P is the same on both sides of the xfmr.

I would agree more than likely the problem is caused by a ground loop.

Jetrexpro,
I can only assume the step-up transformer is an autotransformer.
Could easily be checked with a continuity test.

Unplug the xfmr from the 120V power....

The neutral blade on the 120V plug will have a common continuity with the 240V neutral contact of the 240V receptacle.
At the 240V receptacle, there should not be any continuity between the safety ground contact, of the recept, and the neutral contact of the receptacle.

A temporary test using a ground cheater could be used on the 120V plug of the xfmr just to check for sure the problem is indeed a ground loop hum problem. (Only perform this test if the xfmr is an autotransformer.)

The hum from the transformer is mechanical and not necessarily translated directly to the quality of the AC to the amp. That mechanical hum could be a symptom of saturation caused by overloading or DC offset but some just do that in some places. In other words: Don't shoot the translator. At least, not before questioning the usual suspects.

Saturation also happens when voltage is too high. It is not only mechanical hum but also sinewave distortion.

Thanks Guys!

It turned out not to be the step up transformer. The noise came from some circut boards inside the amplifier itself. When I reported it to Pathos, they sent me new boards, and I put them in and the noise is gone!!