The buzzing is pretty obvious when it is rainy or misty. I passed a power line during my high school bike commute, it never failed to amuse me. I wouldn't call it "loud" though: noticeable, but the average traffic noise is far worse.

The leukemia is bullshit, though. The numbers are too small to make any kind of definitive statement, or to properly compensate for things like socioeconomic factors - plenty of other potential causes on the rural farms and poor urban neighborhoods where we usually build power lines, after all.