What kind of speakers are you using? You should make sure that your pedalboard is plugged in to (as close to) the same outlet as your speakers/amplifier to reduce the risk of ground loops.
The next step is to remove power from your board. If (as it looks) you're just using Eventide pedals, they will all relay bypass when you unplug them, so you should still get audio through them.
Lastly, you can start removing pedals from the loop to track down whether it's one specific pedal having an issue. Basically, the goal is to simplify until you find the one change that makes the hiss go away.