The previous owner weighed in...
"It has always had a thud downshifting right at 40 MPH when in am slowing down going up a hill. Chris (the mechanic) has tried to program the shifting software to eliminate it - but without success. I learned to live with it - but never learned to like it. Sorry.
To my knowledge the torque converter has never been replaced. "

I am experiencing the "thud" at the same time... up a hill, about 40mph.