he is right about the metal expanding and shrinking...new exhaust pipes arent heated and cooled like they are when they are actually on the car. i sell cars and when we get new cars in with only like 5 miles in and have to take them from one dealership to another u can hear when they heat up the pipes and when they start cooling down almost like in an old house when it cools off at night in the summer...its not a really loud knock but a noticible knock right??