This is a Ford 390 (?) in my grandfather's Ranchero. I posted it here because it could apply to any make/model engine I suppose.

He says that when his motor gets up to temp, there is a slight knocking noise from somewhere around the #8 cylinder (rear, pass) at idle (more noticeable) and also when cruising (less noticeable but getting worse). It goes away when accelerating and decellerating. He had someone listen with a steth and couldnt determine whether it was upper or lower end but it is definitely there in that part of the block.

Does this sound like a wrist pin or rod bearing? Cracked piston? What would be the best/next course of action?