SPDIF L and R Chaels popping really badly!? Firestudio project

thanks for the very thorough answer @butchrichard ! so I'm going down your list and I'm at the point of testing what we'd call my 'FS2' or my second fire studio unit. All chaels send out audio, but I'm getting a NASTY LOUD popping/ticking/click thing, it's horrible. It's coming from chaels 9 and 10, which are the left and right SPDIF chaels. Even with the 'power button' off for the chaels, they're still giving me that dreadful pop. What could that be?