Have you called Trane and discussed what all has been done in the 4 years since you have owned it? I would be talking to them. I have called them out for customers on problems like yours and they were helpful.
I would get a new collar with damper.
https://www.homedepot.com/p/SPEEDI-COLLAR-6-in-Take-Off-Start-Collar-with-Damper-for-HVAC-Duct-Work-Connections-SC-06D/202542804
You could get an April Aire power damper that you could wire into the system that would only open when system is operating. They sell them in 4" up tp 12" I think it is.