Having what sounds like a similar issue on my ‘69 S2a SWB. See videos. Rumbling vibration at higher speeds on overrun (not in the video), and a new high frequency rattle at all speeds on overrun. See attached videos. Guessing there shouldn’t be any lateral play at the splined prop shaft joint right?
https://youtube.com/shorts/ciQjcnLda...aurmpIbgaIuWl2
https://youtube.com/shorts/7w8cSNkkG...bmX95h6iTnUxu9