07 Pontiac g5 - bad shifter cable for auto trans?

Hello everyone, I got a feeling that my shifter cable is on it’s way out but I want second opinions before I take my car to have the transmission rebuilt.

So the main issue with my g5 is that sometimes the car will drive forward/reverse and at random, you’ll hear very loud and fast clank/clunking noises which the car would slow to a stop; the RPM needle still moves at a steady and normal pace when driving/reversing. The problem doesn’t happen in Neutral unless you change from drive/reverse. I went to check out to make sure it wasn’t something with the CV joint I replaced and also noticed that when I had the car in Park it moved backwards a way on a flat surface.
The check engine light also does not appear.

Any thoughts on what the problem could be?

If you have the facilities to do it, you could carefully watch the linkages from the shifter to where it meets up with the transmission as a helper shifts the lever. You might be able spot a clue, like a bushing that is loose, a bracket that has come unhinged, etc. But that’s about the best you can do as a diy’er. This is probably going to require a shop inspection to determine what the actual cause is. Suggest to not simply presume this will require trans rebuild. Wait to see what the expert shop guy says. I think you’d do better by visiting a transmission specialist for this. Ask your normal shop staff who’s the best transmission shop in town.

You’ve checked the transmission fluid level, right?