Switch pop fix

I built a Ross Phaser from a PCB. I added a 4pdt on-on switch to switch between 2 sets of capacitors. The switch makes a loud pop when switched. What is your best guess what causes this or how to fix it? I don't have a schematic to share unfortunately.

I've read about pull down resistors fixing pops. Where exactly would these go and is there a good resistor value to start testing with?