I have a pretty big pedalboard, with the H9 right in the middle of it. I use it in pre/post mode. Since I switched to pre/post mode, the H9 puts out a noisy hum that is way too loud, I've checked all of my cables, so that's not the problem. I also went last night and checked every individual pedal, and also checked the H9 in normal mode.. no noise. Anyone else have this problem?
With pre/post it is quite easy to unintentionally set up a feedback loop that might present in this way.
Try disconnecting and reconnecting cables individually to try and see which ones are causing the problem.