Where whould you put a pull down resistor on this layout?

[ylovits]

This is the modern version of the octave multiplexer. link
I made it and it is perfect.. love it, but it makes a poping sound when i switch on and off
Where whould you put a pull down resistor on this layout?

its not the classic "board input to switch input" kind of thing and i am a very very beginer so.. please help out
Thanks

[ylovits]

I tried 1M resistors in both IN Jack & OUT Jack (to ground) but the pop remains