My 2015 451 sometimes does this when I pull into my garage and put it in park. Seems like if I turn off the ignition while still in drive and then put it in park there is no thunk sound but if the engine is still running I get the sound. I know nothing about cars but this is concerning. The car only has 17000 miles on it. Should I be worried? Bought it used from Carmax last year. The noise does not happen every time but it worries me. Thanks for any help.