1999 Pontiac Sunfire maybe its the transmission maybe not

Transmission problem
1999 Pontiac Sunfire 4 cyl Front Wheel Drive Automatic

a few months ago the check engine like came on in our sunfire. According to that little tester they offer at autozone we needed to replace the tempature coolant sensor.

This was a simple to find and replace part. But once the new part was put on every time I put the gear shift into drive its makes a loud banging sound, and when the transmission shifts gears it makes a bang and it feels like a hard shift.

There was never a problem with the transmission or shifting before we changed this coolant sensor, and the problem literally started when we test drove the car after the part change. My husband said some fluid leaked from the line when he took the old part out but it was very little fluid and he replaced it.

We took the car to a garage for work on a different part and when we picked up the car they told us that they thought the car needed a motor mount due to the banging it does when it is put into drive. I am reluctant to believe its a motor mount it just feels like a hard shift in the transmission.

is it possible that it is a motor mount? Or should we be looking at the transmission?