2003 z71 rattling in trans

[btray]

When driving or when rear jacked up sound comes from trans...front wheels or not engaged...shook and listened closly to drive shaft u joints...sound is more forward...sounds sort of like a flang around a exhast pipe or rattleing muffler or cat converter but shook all of those and by the way the whole cat system and oxy sensors are all brand new...please help...it is son's main driver.

Thanks

bill