My 5 L engine knocks loudly when I give it gas. It quiets down when it shifts into 2nd gear or overdrive. It is a smooth running engine. It is a 1988 fuel injected with 50,000 miles. I am told it has solid lifters and therefore not worth to replace as I probably need a new cam. Is this true? I am told i should look for another engine. Should I? Anyone who might be familiar with this problem and could provide some advice, I would greatly appreciate. I would change the lifters and push rods if required if I knew this would solve the problem. Would it be worth while to change the cam and is this a big expensive job... Help or advice thanks....