Owners of 2010 Cadillac CTS vehicles may comment about a slight rock/move 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 rather than felt when viewed from 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 supply on start-up. The 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. To prevent this from happening, the parking brake should be utilized any time the vehicle is parked.
Yeah, try convincing a Caddy owner to use the parking brake.