89 Bronco, 351, 180,000K- the engine seems to run fine, but whenever the engine is running at idle, there is a sound that goes Knock, Knock, Knock, then repeats. It has been getting louder, and my brother (who drives it) says that it sounds like there is a little man with a hammer inside the engine, straight up from the passenger side. When driving, there is no knock, only in idle and it is pretty muffled. My brother is about to go back to college, a 20 mile(one way) daily commute and wants to know if this knock should be looked into before school starts. Any Ideas?