A torque converter if defective will chatter only when engaging or disengaging, you will not feel a chatter/vibration when it’s fully locked up. What you are feeling could be an engine miss fire. If the car is under warrantee , ask for a master inquires for any open campaign letters or recalls. Also ask for any technical bulletins on software up grades.

There was a bulletin on glycol contamination for the 203 chassis, ask if this applies to your car.