Scale; comparing DC cooling to a large building's is like comparing street noise in a suburb to a highway.

It'd be nice to have some hard data on it though. Sometimes the hum is "god damn, that is annoying!" and other times it's someone saying "20 miles that way they built a DC and now it makes the cell phone tower effects twice as bad in this area" when reality was it doesn't show up as audible a half mile away.