This bulletin applies to 2014 and prior GM vehicles equipped with an automatic transmission and auto-start. Some vehicles may rock or move slightly forward or rearward while in Park at start-up after a cold soak.
This condition may be accompanied by a clunk noise. This is a slight movement that is more often seen visually, when viewed form the outside and using the auto-start feature, if so equipped.
The slight movement is due to the residual transmission fluid in the clutch packs that creates a partial apply on start-up. This movement is stopped by the parking pawl and/or transmission fluid exhausting completely from the clutch packs.
This condition is normal and no repairs should be attempted.