I've just put together a new board and everything was sweet until today. I've turned everything on and there's a terrible hum when more than one pedal is engaged.
I'm using a power supply with isolated outputs and the only thing I've done differently is mount everything on a board. Just mdf board with Velcro carpet. It's doing my head in how it can go from working perfectly to sounding terrible. From what I can gather it's ground hum, at least I think so. If I put my hands on the strings the hum stops
Ground loop hum, probably from your power supply. Unplug the pedals one at a time til you find the one that caused the hum then check it for a faulty ground or power supply issue.