popping noise

Does anyone know why there's a popping noise coming from a 2003 chevy ext.
cab after driving it and parking after a few minutes while standing by it
you can hear a loud "thump" coming from the truck. The truck only has
25,000 miles on it and was a GM certified used truck when I bought it it
only had 19,000 miles on it. About 2,000 ago the transmission just went
out.. and had to have a NEW one put which I must say GM or dealer would
not offer any help on paying for it..after that this noise has showed up.

removing and installing the cossmember while vehicle is on a post type lift
can cause improper settling of frame causing pops and squeaks.best to
loosen crossmember and trans mount slightly start up vehicle and shift in
reverse and drive then back in park to settle drivetrain. shut off vehicle
and retighten crossmember and mount.