electro harmonix true bypass problem

[arielfx]

i sold some EH pedals (dlx memoryman, small clone, polychorus) to a guy and his amp (hiwatt) would pop when he switches them on. i looked at the switches and even replaced one just to make sure but there is a slight pop when pushing the button. it's not the HUGE POP when you don't have pulldown resistors

what can it be and what i can do to help him?