Take the cover and see the side of the box where the two panel (top and the bottom) is get contacted, scracth the surface with smooth sandpaper and take back the Cover, take back the screews and mybe all gone like in my XL7.
this is fix my click and pops for years now,
i get this problem in different ways when i use spdif output, my soundcard has one broken diode and create clicks and pops.. im fixed and now that click ar gone...
hope your gone too