I’ve swapped pedals, cables, power sources. I’m stumped.

When you insert your stompbox into H9’s Pre/Post loop, does toggling Pre/Post affect the noise? What if you bypass the stompbox while toggling Pre/Post? 

I don’t know…If H9 works fine on its own in either Pre or Post, I don’t think it’s H9’s problem.

Again, I appreciate the help.

When I insert any stompbox (OD or Delay or Tuner) into the H9’s Pre/Post loop I get the hum regardless of if that stompbox is bypassed or not. Switching between pre and post doesn’t change the level of the noise.

Yes, the H9 works fine when I connect just the pre or just the post side. But the noise consistently happens when I connect both sides of the H9 regardless of what cords, stompbox, and power supply I use in the loop.  The noise always goes away once I take the second instance of H9 out of the chain.