Are you using a midi controller to switch channels? Is it the case that the modules all come on when you hit a switch for an effect or the like?
There is a special switch for programming the RM4 to switch based on midi settings. So maybe it was pressed in by a cable or wall at some point while you switched through the channels and they all got set to come on at once. If more than one channel is on at once, I would expect there to be crackling. I accidentally turned multiple modules on at once with my setup and got the crackling. Also, ther is a set of MIDI switches on each of my M4s that can be thrown to set up how switching occurs, which might affect default channel settings or allow multiple modules for channel mixing or similar. Maybe these switches are also present on the RM4s. The manual shoud have info on these types of things, I expect.
In any event, I look forward to hearing about your solution when you figure it out. Good luck.