2nd Shift and 3rd shift noise(Knok knok)

[zahidismaeel]

hi guys

I m experiencing a knok sound(Automatic transmission) when drive at morning(first time) or drive after keeping car switch off for 15 min.
I went to my local GM dealers they have checked the transmission infact they offer me for test drive with their QC(quality control) person just to assure me that nothing is wrong and computer log is errorless but still a strange sound is there..
For sure some metel piece is creating a sound but unfortunately they dont know how to fix it :( just they focuing on what is coming on the computer and thats it.
Anyone have idea about this issue or someone is experiencing like me.

Zahid