How to get rid of transformer hum?

I realize that you have measured a 5% sine dip. But are you certian that this is the cause of the hum? I have tracked down a lot of hum problems and most are generated from within the home. I will relate my own personal experience. I always had a slight hum in my tv and Tice isolation transformer for my gear. I lived with it until I bought a Classe CA400. The amp hummed very loud! I finally isolated the source to my heat pumps outdoor unit. Something in the prehaet circuit of the compressor. Right now I just turn it off when I listen. The background is dead silent! I'm going to look into it deeper when I get the unit pm done. I would try turning things off in the house first to see if the hum is caused internally. (off means off at the breaker) Good luck!

A while back some funny Agon poster suggested that the way to stop an amplifier from humming was to teach it the words.

PS: As far as using soft footers directly under the units, I disagree with Steve's comment. I have an external power supply on a Musical Fidelity amp (that sometimes hums). Using a soft footer in this manner just isolates the vibration to the unit which deteriorates the sound in this case. I have found it much more effective to draw some of the vibration from the unit to a Maple platform with the use of cones. However the platform is isolated with soft footers (Vibrapods) so that other vibration is not introduced to the platform. I will be simplifying this setup with the use Neuance shelving, but still recommed the use of cones/platform/soft footers as an inexpensive fix on amps and power supplies.

If it is a DC bias then a isolation transformer may help. If the isolation transformer wants to hum because of to high voltage then it may have a higher voltage tap you can try. You could also move the isolation transformer to the breaker panel. I have a 5kVA unit that I intend on wiring tothe breaker panel and then put my dedicated outlet on it.