Troubleshoot the source of humming. Plug guitar into amp, same with fiddle and see if the noise is there. Try the same thing, but with the pedal in the chain. Humming generally comes from an amplifier issue, but the intermittent crackling could be an entirely different issue.
A more quality way of summing his inputs going into the pedal will certainly clean up his signal. Especially since the impedances between the electric guitar and fiddle are most likely way different.
The noise hasnt been as repeatable as I would like - seems to start in the middle of soundcheck (when ironically I have the least time to troubleshoot) - and then it seems to disappear once everybody is on stage and playing.
It sounds very electro-mechanical - swells up and down.
I should also mention we have 5 wireless units going as well - all different brands.
Most recently - this happened at the York Toyota Expo - except it was just in the monitors, the mains were clean...very nasty...the performers really had to suck it up for that one. When you spoke into a mic - it would have a electrical resonance - kind of like a feed back ring...except it was electric hum and not sound.
More or less the same sound on 3 different PAs...although the recent gigs at our home theatre seemed fine.