i took the car for a drive and it has a 250 inline 6 and the engine strted knocking i think its a bad valve but all of a sudden the exhaust got lower and louder just posting for some opinions.
Check and see if you have an exhaust leak...Check manifold...Pipe flange at manifold...ETC... Exhaust leaks can make odd sounds sometimes...Good luck!!!
Can you tell if the knock is from the upper or lower end? That may help you narrow down whats up. How many miles are on it? Could be so, so many things.