when I am driving and I want to stop I have sound like knock from the gear.

Hot gear oil will not cause the knock. Curious; how did you measure the gear oil temperature? I assume something's loose or damaged and moving. Make sure you don't have anything in the car or trunk that could be moving around including the spare tire and tools then try another dealer, I seriously doubt it's normal. If they say it is then test drive another one to confirm. A large percentage of the noises I diagnose on peoples cars turn out to be their stuff in the car moving and hitting something, I've found things from bowling balls to flashlights and occasionally seatbelt buckles or other parts of the car making noises that went undiagnosed or misdiagnosed at shops.