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My 2007 Mazda CX-9 recently developed a knock when going from Park to Reverse after the vehicle sits overnight. It doesn't happen at any other time during the day.  It also doesn't produce the knock when shifting into any other gear.  Any suggestions would be appreciated.  

are you dropping it into reverse with the engine cold and at fast idle ????/

cold starts the engines need to run faster to warm up various emission devices.. like the cat and the oxygen sensor...

it will except when it is really really cold.. come down off fast idle in under a minute...