Well I have a 89 IROC with a 350 all original 117,xxx miles. Sometimes when I start the car and it is sitting their idling I hear a really loud pronounced metal to metal noise from the engine like a quick hit of a hammer on a solid metal object. Its not constant and maybe repeats every 40-50 secs. It tends to happen more when its cold out. The noise isn't there when I'm driving or I can't hear it any ways. After a drive if I let it sit and idle it does not have this noise. I am thinking it might be a valve sticking open or a spring going bad and the piston is closing it??? Any ideas