This just recently happened. My SCA DI and 2 N72 models have been working perfectly for about 2 months now.
Yesterday I was going to play guitar through the DI and N72 and all i heard was this horrible hum and poor signal coming through. I tried different cables and completely removed all connections from the patchbay. I've unplugged everything inside the chassis and tried to narrow it down. It seems to be some grounding problem but it does it with any combination of the modules plugged in. So I'm thinking maybe the power supply? When I have a cable attached to the N72 output I can thump the cable and hear it. Also, you can hear the clicks of the switch when it is being moved as well as knocking on the chassis and hearing that.
Any help would be greatly appreciated.
Thanks,
Ben