E6 inconsistent knock

[Lukasswaziek]

Just bought a "new" truck, an 81 with an e6-350 and I'm starting to use it, I have been hearing an unusual knock in the front of the motor at idle, it sounds almost like two marbles hitting each other, or a hard wood block being hit, and it is inconsistent, or it makes noise at random intervals. Not a constant repeating noise. I first thought it could be a bad harmonic balancer so I replaced it, but no change. Is it possibly a fuel knock? It doesn't seem like it is because I can only hear it right in the front of the engine, possibly the geartrain. I don't want to go too long and destroy an engine, but I also don't need to tear it apart if it's minor. Help would be appreciated 

Edited April 17 by Lukasswaziek

[Joey Mack]

have you taken the belts off and run it? 

[fjh]

If the engine has an AMBC fuel pump You could try  and extra Gasket under the fuel transfer pump ! The little pump in the main pump tend to put side thrust on the pump cam and Can change the timing on number one cyl as the bearings wear! 

just something to try! 🙄

[Joey Mack]

are you hearing the air compressor kick on and off? 

[Lukasswaziek]

No it is not. That's pretty easy to pick out from everything else

[Joey Mack]

any chance the thrust buttons are out of adjustment and the cam is walkin' a bit? 

[mack88]

My E6 4V did it to but sounded like from the side intermittent at idle was doing it for couple years,Don't recall hearing it after new injection pump. 

[Lukasswaziek]

I take the belts off and run it and it seems like it goes away. But alternator is new and I've never heard a water pump clunck or knock like that. Anything is possible I guess🤷

[JoeH]

If it continues, I'd pull valve covers and double check valve lash. You could have a dropped valve seat.