Popping in stomp box effects is primarily caused by imperfect input and output capacitors (used to block the DC voltage used to power the pedal) that leak. This popping can be fixed by using a pull down resistor (between 100k and 4.7M, exact value is not critical) from the outermost lead of the cap to ground.
- Geofex - Why does my stompbox pop when I hit the bypass switch? - Causes of popping and how to fix them
- Geofex - The Ins and Outs of Effect Bypassing - Also has information on popping
- Re: Is mechanical true bypass switch popping just an inherent flaw?