Electro Harmonix Poly Chorus - Loud pop on main output

[ainokami]

I just got an electro harmonix poly chorus.
It sounds great and works fine, except that when using the main output, when I stomp on the 3dpt switch, there's a loud pop.  The pop does not occur if i connect the pedal to the amp using Output 2.

I opened up the pedal, and looks like the 3pdt wiring is not as usual. Looks like  input jack is a mono jack, and also, i dont see any cable going from ground to the 3dpt switch...

Any suggestions?