As mentioned check the converter bolts.
Once the Converter bolts are tight one may also be able to see / check if the flex plate bolts are loose at the crank by trying to move the converter CW /CCW.
Rotate the motor around and check the flex plate for cracks preferably watching it while bumping it around with a remote starter switch.
Verify the engine mounts have not collapsed
Check the entire exhaust system for any place it may be touching anything.
Check for anything on the engine or trans that may be touching the body anywhere.
A real longshot would be a rotating heat riser butterfly
Would be very helpful if you could do the above on a 4 post lift
keep us posted