Out of curiosity, have you tried monitoring off the UA-5 to see if the sound is definitely happening between it and the jb3? One time recently, I was running my UA-5 at 44.1, to a Sony D8, and I could hear this popping noise while in rec/pause mode. It went away if I switched to 48khz. I've never heard it since on either sampling rate. Sometimes weird things happen. *shrug*