Ok so I'm still getting the bugs worked out of my 505 based 440.

Finally running great so I took it out cruising yesterday. While driving on the highway for approx. 10 minutes, I heard a faint grinding noise, so I slowed down a bit. Then I heard it a few more times. I shifted to neutral and pulled over and shut it off. I then restarted it and I could only get it into first without grinding. I was close to the exit so I idled it off and into a parking lot. Nothing out of the ordinary underneath.
With the car off I manually shifted thru all gears including reverse.When I restarted the car, everything worked great and I drove it home as normal on the backroads.

Any ideas as to what's going on here? I'm not sure if it was trying to engage another gear while driving or what