Loud drivetrain noise

Hi everyone, I just purchased a 97 legacy gt, when I drive at highway speeds and let off gas pedal it makes a loud ratcheting or grinding. Noise when I accelerate it goes away. Only does it at 55 and higher speeds . Sounds like around transmission, it is a auto transmission that was just replace bye a service center, used trans, and driveline used, there is a little ujoint wear and movement on the ujoint closest to the trans. The previous owner just got it out of shop this week but didn't drive it on highway. Before selling it to me. And I didn't drive it on highway until after I bought it. Previous owner seems honest and claims its under warranty for 3 months. I'm taking back to shop next week. But I need an idea of what's going on so I don't get jerked around. I also checke heat shields, they seem to be fine I did take cat shield loose and remove some small rocks , but didn't seem to matter. Please help if you can

If it was replaced with all used parts, it can be anything. Sorry I couldn't give you an answer but this is really tough since its so used.