Noticed a noise today when my wife was backing up from driveway so once she returned I had her do 1200rpms in park and couldn’t replicate the rattle so had her put it in Drive and hold break and do the same rpms and boom could here the rattle what could this be? Also made the same noise in reverse and tried in neutral too with no noise? Tried crawling under vehicle sound seemed to be coming right under engine.

2012 suburban 145k 5.3

 

Sounds like an automatic transmission issue to me. 145K with no fluid and filter drops will do that,