After working on my car for a few days it is finally running. It has a bit of a rough idle and some sort of strange knocking. It is an 84 z28 5.0 h.o. I have no idea what it is. At first I thought rod knock, but it is only there at idle every now and then, then goes away, then once every now and then but only at idle. Excellent oil pressure, and runs beautifully when I took it on a test drive. No leaks from anywhere and it seems to only be coming from the very top end like the piston is hitting a valve every now and then. So I pulled the heads off, checked them and no damage. I am really stumped because as far as I know about rod knocks is that they are constant and cause problems with oil pressure and a few other things. Could the noise be due to old gas? The car sat for over 3 years, and I put gas in the tank not knowing if anything was actually in there because the gas gauge does not work. any ideas?