This is sometimes caused by dirt or dust collecting underneath the silver knobs. Two suggestions:

- vigorously move the knobs back and forth from min to max positions many times

- 'park' the knobs when not using them for editing. This means moving the knobs to the min or max position and leaving them there while playing. The knobs seem less sensitive to detecting vibrations at these positions. Even though you don't think it's vibrations because you've taken care to have a quiet stable position - it might still be caused by dirty connections.