[Chris Burkard]

If it truly is coming from the prop shaft, then it can only be the cutlass bearing, shaft seal assembly or the shaft coupler. I'm assuming you checked the props? I have seen loose props make a rattling noise. I know you said the shaft was checked for straightness, but have you had the props balanced recently? My suggestion is every 5 years if you don't hit anything.

The other drive train issue that can cause a rattle like sound is the transmissions or motor and transmission mounts. These issues can actually travel down the shaft and make it appear the noise is coming from the shafts or props.

If you do figure it out, post it so others can learn from your experience.