Usual sounds

Hello everyone, I'm new the the tundra world and I must say I'm glad I switched from the silverado.
I'd like to hopefully get a few opinions about this unusual sound iv been getting.
I have a 2020 TRD Off Road with 67,000 stock. I only added a external trans cooler with a OME thermostat.
Iv had it for about 10 months.
A few times after sitting when backing out of my garage I here the strange metallic sound, the best way to describe it is like a BB's rolling around in a metal container. It will continue forward down my driveway for about 100 feet then go away.
I'm all up to date on all my PM
I haven't taken it to dealership for this issue yet because I'm afraid of it turning into a witch hunt. Unresolved issues it partly why I turned my back to GM, So guess I'm kinda hesitant to open that can of worms until I have a better idea what my problem is.
Any opinions are appreciated, thanks.

 

Welcome from LA (Lower Alabama)! I would have a buddy stand outside the truck and try to identify where the sound is coming from. Or if it's in the cab, have your buddy sit in different seats trying to locate the source while you drive. Then go from there to a resolution.