If transformer in DAC behaves like one in power amp then read from Spectron Audio Web Site:
"Mechanical humming from the mains transformer is caused by a varying DC component on the mains grid's AC voltage. The DC component comes from unbalanced loads on the mains grid, such as hair dryers and is present in most households. The amplifier main transformer's vibrations degrade performance of the power supply and when audible can be annoyance as well. Spectron solved this problem by building appropriate filtering circuit below one watt, which is extremely low."
I had it until Spectron changed design of its amp to include appropriate circuitry. You can buy external devices which filter DC component out: one was sold by PS Audio...do search.
All The Best
Rafael