Behringer xenyx 1832fx Fx crackling!

[diydidi]

Hi All!
I have a behringer mixer on the bench for repair. Problem is that when you turn up the fx on a channel, it crackles/ticks along with the audio being on input. IOW it only crackles when there is audio present. I have checked the 5V line, and its quiet. SMPS seems ok and stable! I'm also sure its not the neighbor's electric fence through the mains wiring. The ticking sound heard is a fixed 2Hz or so. All the fx patches sound and work fine!!
Any ideas?

[Jazz P Bass]

There is an FX out jack. L & R are the tip & ring. Sleeve is ground.
They must make a normally closed condition.
You could try monitoring the jack to see if it acts up there.