The buzzing will be the transformer and the common reason in this case is that the transformer was probably designed for UK power supply, but with a 120V primary tap for selling in the US. Very few manufacturers use a different transformer for the different mains supply. So being a British piece of gear the transformer is probably optimised for 50Hz operation, yet in the States you use 60Hz. Usually this does not present much of a problem and the situation is usually worse the other way around - because 60Hz operation is usually easier on a transformer than 50 Hz is. My local mains is 240V 50Hz, and my Theta Gen Va DAC buzzes badly because the transformer does not like working at 50Hz. But I would not fret about it unless the noise is intrusive, or the unit is getting unusually hot. I have found that using better isolation devices such as seismic sinks or BDR cones reduces the buzz markedly. You could also apply some damping directly to the transformer.