This bulletin applies to 2014 and prior GM passenger cars and light-duty trucks. Some vehicles may rock or move slightly forward or rearward when 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 visible rather than felt, when viewed from the outside and using the auto-start feature, if so equipped.
This 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. This slight movement has no adverse affects on the transmission. To prevent the condition, the parking brake should be applied whenever the vehicle is parked.