[Marshall V. Daut]

Years ago the cluster gear shaft in my '31 Roadster broke in two. Why, I have no idea. Probably a cheap imported shaft from the 1970's or 980's that came with the car when I bought it in 1984. The broken shaft allowed the cluster gear to move up and down slightly, as the two shaft halves flexed at the break. That caused noise and sometimes shifting problems. In Phoenix where I drove at the time, I didn't encounter any inclines that would have possibly caused the cluster gear to tip downwards in the back going uphill. I can imagine that had the gear changed angles like that - even slightly - it may have bound itself up against the small high gear up front on the main shaft, but laid back down again on level ground. Then the transmission could be shifted, but it still had some growling noise. Outside of that noise (which you didn't mention), this could be what's happening inside your transmission.
Without fully tearing it apart, you might be able to slip a looped wire underneath the cluster gear, grab the one end and pull up so that the gear is "cradled" in the loop. Then pull up on the wire to see if the cluster gear moves. It shouldn't. If it wiggles up and down even a little, it may be caused by a broken cluster gear shaft or even one of the two roller bearings inside. Check this out before panicking too much.
Marshall