I have the 305 tbi engine in my trans am. The car sat for 5 years before I aquired it. I have run seafoam engine additive and changed the oil four times. The oil looks good when I change it as do the filters. However the lifters still chatter a lot upon a cold start for about five minutes and then the noise goes away and the engine sounds really good, no unusual noises at all. What do you suggest I do?