Per your description, I would suspect that it is RF feedback into the rig. Try transmitting into a dummy load. I had the issue with my off-center feed dipole. Had to ground the rig better, and then put a common mode chock on the coax cable feeding the shack. That did it for me.
As to the volume control, I would send an email to HF Signals explaining the issue and request a replacement. It will mean removing the control from the boar and replacing it. I do not have a v6, so I cannot comment on how difficult that would be.
Other things to try:
- inspect the solder joints of the volume control with a magnifying lens. Touch up any that look suspicious
- try spraying the volume control with contact leaner and see if that helps.
The above are just suggestions. Others may have some to add.