My 2012 Impala with a 3.6L VVT engine and 6 spd trans has a loud knock (Helicopter like noise) in the engine when the vehicle is warm and accelerating from a stop (Low rpms). Sitting in my driveway with the vehicle in gear I brake torque it, rev the engine slowly the knock starts and at around 1800 rpms you can hardly here it. It does not do it idling in park or when it is cold. Anyone have an idea what it is?

 

Might be a motor mount.

 

It's a repetitive knock not a single knock as the car accelerates.