Tranny rattles. But when I accelerate it stops. It only does this when stop/go. on 2001 Buick LeSabre

My 01 Buick lesabre has a rattling noise coming from underneath it. I had the tranny fluid and filter changed what could be the cause of the rattling sound?

2 answers 2 comments
Most likely broken/loose catalyst inside the converter! (Catalytic converter) Or loose exhaust heat shield.
Thank you so much
agree with #1 or poss cracked flex plate
Thanks you guys were helpful.