electrical question for "y'all"

I had 2 guitar pedals connected on the same cable from an AC-DC converter, and the output when passed through both pedals in turn had a loud 100Hz hum added to it. I took them off that power cable and onto 9 volt batteries, and the hum was gone. So, 2 questions: (1) Any idea why that hum was getting through the conversion to DC power, and (2) Any idea why it was 100Hz and not 50Hz? Did the 2 tones just happen to be 90° phase shifted relative to each other or something?