Loud pop when selecting playlist or switching to a

I have upgraded firmware to the latest version. I assembled two new list and did the export as normal. Got to rehearsal and our drummer powered up as normal then we decided to rehearse our upcoming show and he selected the one of the two playlist for this week. When he did this a loud pop came through the PA and scared the hell out of us. Since I man the mixer during rehearsals I asked if he could go out and select another while I lowered our main slider and so I can watch which channel made the pop from the LP16. We use the following setup on The LP16 ch. 1 is a bass track, ch. 2 is a background vocal, ch. 3 is a click to the drummers mixer for his ears, ch. 4&5 are other instruments. We checked each of the 1 – 5 outputs and it happens every time you select a playlist on all outputs. This only started after the firmware upgrade before that it has been rather flawless. i have gone back in and re done each set and again at last nights rehearsal…LOUD POP when selecting any list!!! we have 3 big shows this weekend and FOH engineers will hate this @#$% please help ASAP.
Thanks as always!