I have a suspected rod knock, and when pulling the plugs with the engine running the noise stops when cylinders 3 and 4 aren't firing. Is this definitely rod knock, or could it be something else? Should I bother getting it diagnosed at a shop or is it done for? Also, the fresh oil I refilled it with after it ran almost dry (the cause of the problem) is already very black and the engine has only been started 3 times since the oil went in. Am I screwed?