Thanks. It's an 81 Berlinetta V6 that I bought from the original owner 10 years ago. I swapped in a 5.3 and a 4l60 and it's been on the road about 5 years. I just swapped in a t56 and it's like a whole different car. Still need to finish the interior.
I had a similar noise that sounded like it was coming from the back of the car and was driving me crazy. Turns out it was the parking brake cable rubbing on the exhaust. Maybe try opening the decklid with the car running and see if the noise changes or goes away.