clicking noise - need help

I do have a ht system with pre, sub and amps and a stereo pre integrated by unity gain.
now i wanted to change subwofer-setup from processor-lfe to line in, using either amp-loop by my mc452 or second main out of my arc ls-27.
in both cases there is a loud clicking noise when switch mute on/off or channel of ls-27 pre.
anybody s got an idea?
is the unity-gain loop the problem?
using lfe of processor works perfectly without noise. but i would like to support deep bass by sub with my stereo
subwoofer is velodyne 12+
Time to clean your integrated under chassis including cleaning and treating selectors, switches and potentiometers
If that doesn't help, you'll need to service and replace noisy DC caps.
the issue is only with balanced inputs; unbalanced is no problem, so i use rca now...
than it's probably dirty or loose XLR recepticle(s). i'd put some fresh solder over the xlr terminals and clean pins on both recepticle and cable.