mashedmitten wrote:DC riders are one part of it, but a mangled sine wave is another.
Right on mashedmitten!
I've certainly noticed this transformer hum as a result of a distorted mains waveform (I even went to the trouble of wiring up an old 9-volt AC wall wart and a couple of resistors as a potential divider so I could plug the mains waveform into my audio interface and examine it more closely when ever it happened (don't try this at home folks unless you're REALLY sure what you're doing ;) )
Odd harmonic distortion in the waveform (flattening of the waveform 'tips', typically resulting in around 1% measured THD in the mains 'sinewave') didn't result in any transformer buzzes.
NO TRANSFORMER BUZZ - 1% ODD HARMONIC DISTORTED MAINS WAVEFORM
However, there was a loud transformer buzzer in my power amp as a result of even-harmonic distortion (even though the THD didn't rise any further), with visible discontinuities in the waveform around the zero crossings.
LOUD TRANSFORMER BUZZ - 1% ODD & EVEN HARMONIC DISTORTED MAINS WAVEFORM
Having said that, these buzzes came and went as the waveform changed - 95% of the time I don't hear these buzzes, even during brownouts (I once measured one down to 179v)