Have you checked the hangers on the exhaust? They are a bad design and rust off - if the exhaust is not suspended right it can do this - we had it once and it was very intermittent - a knocking noise - only when the exhaust got into a certain 'momentum' that it did this - although it happened every time we went over a certain humpback bridge and the 'bounce' caused it to rock in a certain way. Thought it might be worth mentioning
