I've had "that diesel noise too on my last truck too. It was a crack in my header. this sounds different, or from a different area,
I think.
I'm going to record the sound see if it comes out decent,
maybe thatll spark some ideas.

I saw an engine stethescope at harbor freight, that may be worth picking up.

BUT
Do I have to take the head off if Im going to get to the chain gides and damper????

I'm givin up my toy and goin VWesty. Got my Westy and another Toy:)