Mark IV FX loop hum

[Fronzil]

I've just noticed this for the first time..anytime I have cables going into the send/return in the back of the amp, there is a slight, slight hum going on. doesn't matter if there is even a pedal in the loop, always does this (loop on only, goes away with connected cables and loop off)..i've always used the loop so i've never really noticed it coming on and off..today however I took the cables out of the send/return for some pedal shootouts and noticed it....anyone else notice this? I only run a tuner, BB preamp out front and EH DMM in the loop, all powered by the PP2+ with plenty of juice left to spare.

 

[THTH]

Just tried this with a MKIVB combo at low and high volume and only silence.