This is an odd symptom. I’ve never experienced this before. But it has to be something simple, like a bad connection or solder joint…there’s not much after the volume pot except the output jack. It’s hard to say much without hearing it myself. If you could send me a cell phone video showing me this problem i might be able to have some more insight.
Before you send the video tell me what circuit board version do you have? 6.x or 8.x?
If you have board version 8.x then see what happens when you remove (or lift one leg of) C17.
If you have version 6.x then proceed with the video.
Our email is info “at” zeppelindesignlabs.com
-Brach