Reversed Mono Jack Leads

Hello all, I stumbled upon a solution last night that has me curious. I built the Nurse Quacky effects pedal on a breadboard and it sounded great except for a DC hum (at least thats what I thought it was). I tried different RC filters to see if I could get rid of that. I ended up trying a low pass and a high pass filter and they both had the same effect (hum completely gone) and that seemed odd so I just tried reversing the leads of my mono output jack to see if that was the culprit and it completely got rid of the hum. I was wondering why reversing the leads would cause a hum like it did?

No more ground loop..... Careful if you use those two circuits again because one's ground is the others VCC.

The RC filter I used wasn't in the Nurse Quacky circuit when I fixed it, I just switched the output leads from the Nurse Quacky itself. Only one circuit when the hum was present. Maybe there is still a ground loop in the Nurse Quacky circuit. Hmm.