Knock at Idle

[Ian Curtis]

I have a pronounced knock at idle, which goes away when you advance the spark. (Not sure if the sound is going away, or just being drowned out by the engine noise) It also goes away if you short out the #4 plug with throttle/spark at idle and retarded.. What do I need to do next to diagnose/fix the problem?

Video of knock (beginning is retarded, after I go in the cab is advanced)

Thanks!