i have a 1990 f250 7.3 idi non turbo. I just got done re doing the head gaskets on it everything has been going great for a couple days. But I notice it has a knock when first start til oil pressure comes up on the gauge. It never knocked before is the oil going bad? It still has four injectors that need replaced. If you start it up it will knock til the oil pressure it up then if you turn it off and back on it won't knock. I'm confused on what to look at.