Return vent in unfinished basement causing first floor to become pressurized
Your main trunk duct may be undersized for the return air on the main level. If this is a new system that used the existing ductwork the fan speeds may be set for the highest efficiency.
In my opinion the system is not properly sized or set up correctly not enough info but one of the 2 is true.
For a fix:
It looks like a Rheem but not sure, the Rheem furnaces / air handlers I have installed have multiple motor speed taps. Running the system at a lower speed should help reduce the pressure imbalance.
Adding a larger intake trunk duct to pull air out of the upper floors would be a option since there are already 2.
If you add heat to the basement this will eliminate most if not all of the imbalance but I would absolutely want a damper on the supply duct for the basement so you can balance the flows or much the heat / ac will end up in the basement.
Even a single heat duct down there may reduce the pressure imbalance.
I would check to see if you have a multi tap motor that could be run at a slower speed if you are not ready to add heat downstairs.
I just re read and the system is 3 years old so you are past the point where you can usually get something done for free.