I didn't actually check the battery mounts (yet), I didn't mess with the battery during the VPG job other than to disconnect the negative terminal. I do think you are correct in that it feels/sounds like something loose and shifting, something fairly heavy as well.
I did recently noticed that the lower portion of the back seat was not properly clipped in to the retaining points, so I fastened it in, but can't imagine that is the source of my current issue. The investigation continues...
