I have a 327 in my 59 chevy. Last week when I got home from a 60 mile freeway trip I heard a horrible knocking noise coming from the engine compartment at idle but only when the car was in gear. I drove to my friends house who is also a mechanic to listen to the noise. When the car was in neutral or park the noise was not there, but in gear he heard the noise and identified it as the alternator. If you put you pull lightly on the alternator the noise went away. He thought it was a bearing inside the alternator, so today I put in a new alternator. As you guessed, the noise is still there. If I pull the noise goes away. So before I start chasing a problem throwing new parts at it, any ideas what could be causing the noise?