because you say your engine has a lot of miles on it, it could be a rod. also theirs no telling how the previous owner treated it. possible reasons could be a rod, fuel injection problem, bad timing belt position, faulty cooling system, ect.. my advice is if the car means a lot to you then take it to a local shop, get a diagnosis from a certified technition. just to be on the safe side. but a knock in the engine will lead to serious problems down the road, if not taken care of soon.
hope this helps
Terrell