2006 Chevrolet Silverado 1500 Pickup Owner's Manual - Page 119

PARK (P): This position locks your rear wheels. It is the best position to use when you start your engine because your vehicle cannot move easily. If your vehicle has the Allison Transmission®, with the Power Takeoff (PTO), use PARK (P) when starting. The Allison Transmission® has a park pawl that is designed to hold the weight of commercial medium duty trucks. Therefore, the Allison Transmission® uses larger parts than other automatic transmissions. These larger parts may sometimes make a clunk noise when the selector lever is moved from DRIVE (D) or REVERSE (R) to PARK (P). This is normal and there is no damage occurring during these shifts. When parked on a hill, especially when loaded, you may notice an increase in the effort to shift out of Park. See Torque Lock (Automatic Transmission) under Shifting Into Park (P) (Automatic Transmission) on page 2-50 for more information. {CAUTION: It is dangerous to get out of your vehicle if the shift lever is not fully in PARK (P) with the parking brake firmly set. Your vehicle can roll. Do not leave your vehicle when the engine is running unless you have to. If you have left the engine running, the vehicle can move suddenly. You or others could be injured. To be sure your vehicle will not move, even when you are on fairly level ground, always set your parking brake and move the shift lever to PARK (P). See Shifting Into Park (P) (Automatic Transmission) on page 2-50. If you are pulling a trailer, see Towing a Trailer on page 4-67. 2-25