Clunking Noise problems of the 2011 Chevrolet Silverado

#1. Vehicle accelerates when letting off gas, even while going uphill the vehicle will accelerate. You can go up a steep hill without touching the gas. Brought the vehicle to the dealership they replaced, the throttle body, gas pedal assembly. They reinstalled the original parts and told us that this was normal. We drove other 2011 chevy Silverado's which did not do the same thing. The dealership told us that all the 2011 chevy Silverado's accelerate on their own. #2. There is a clunking noise from the rear when driving over bumps. The dealership said this is normal backlash, and did not do anything.