I need some help diagnosing a problem. I have an 83 with a 305. Whe it idles, it sounds perfect, then it knocks a few times, the idle drops, then it picks back up and runs smooth again. It seems like it loads up, drops idle, then it goes back to normal. Any ideas?? I put the stethoscope to the drivers side valve cover and it sounds like it's coming from there. Pushrod?? Rocker arm?? Help!!