FWIW, I'm running digi-in and this is definitely different than the noise encountered running analog-in to the JB3. As for age of the unit, this was the inaugural run for the JB3 in question - bought it a while (a year?) ago as a backup, but broke it out to run two simultaneous sets of gear this past weekend. And yeah, it's possible I jostled the gear in the timeframe during which the noise occurs. My immediate reaction was "someone's cell phone" or RF interference, but that wouldn't explain the pattern of the noise nor only occuring in a single channel on a single deck. I don't think we're going to resolve this one anytime soon, so I'll just have to monitor my gear performance closely for a while to see if I can nail anything down. All - thanks for the discussion and input, I'll drop a line if I encounter the problem again.
I'd say it's entirely possible that the noise was induced, because it's one channel could have been how one particular cable was coiled, where it was etc. I did car stereo installs for quite some years, and tel-com work now, you'd be suprised how many factors can contribute to induction on a conductor. Who knows, it might even be something simple that occured in the UA-5 itself or the JB3 capturing and writing the dig signal.
Maybe some day we'll nail it down, but I'm guessing you'd be just as happy if it was just a one time anomaly.