This has been going on for a couple of years now and i still can't pin point it. When the car is first started it has a miss in it until it idles down. It could be mistaken for a knock but I dont think it is. I have tried to rule this out because I store it every winter and the first time it gets started after sitting for 4months there is not a bit of valvetrain or bottom end engine noise so thats why I dont think it is this. This is the only time I dont experience it. I was wondering maybe a fuel pump? I know these things get tired and having over 100k on it that could be a cause. Any ideas?