AZ.... That's exactly the kind of noise and the way it feels in my car. The noise comes from behind me and you feel a faint bump through the floor. It's usually just one pop unless I stop really guick from very slow speed. Like just rolling forward and slam the brakes. Then you can hear and feel the sloshing.

But, in your case, since you just had the tranny replaced, I think I would take it in and make sure everything is torqued correctly. Maybe it's a loose bolt or something related to the tranny swap.

Good luck!