If it is still SUA it would only rotate under a healthy launch or load, like leaving the stop sign with boat in tow or hole shottin it, IMO if at all. How many miles on that driveline ? If you speak with Georg ask him if he has found a source for new oem slip spline or what he replaces them...
I get it, phasing is simple but in my experience if its outa phase it rattles constant not intermittent. I would lay money its the wear on the slip spline.