My 96 S-10 has 260,000 miles and a noisy rattle in the top end. I have replaced the pushrods and rocker arms but the noise persists. It sounds like a low octane knock but it occurs at idle and whether under load or not. I have heard the same noise from several S-10's of similiar years. Any ideas???

 

It might be your injectors firing they can be noisy some times I had a T -bird with noisy injectors it drove me crazy for weeks before I figured it out.