Pork Barrel ticking - variables

[Strategy]

I built a Pork Barrel and observed a behavior with the ticking that's new to me. When I use a line level audio source (Farfisa organ, synths, drum machine, etc) there's no ticking and when I use guitar as the input, I get some minor ticking.

I'm sure the science behind this is more simple than I'd expect, but can't figure it out - any insights?

Strategy

[juansolo]

Ticking on the PB is usually internal cabling straying close to the clock. If it can't be avoided shielded cables can help.