engine knock at idle, but not under load ?

[The motor guy]

I heard a fedex truck driving yesterday and it stopped at the stop sign and it had a horrible knocking sound, but then once he got on the throttle it went away and sounded like your normal cummins turbo diesel. I believe that's what they put in those trucks, that's what it sounded like. I suppose it could have been a valve train noise, but it really sounded like a knock. What do you think it was?

 

[MrkofXcellence]

One of the Freightliners that is in a school bus when it drops kids off at the school where I work sounds like that at idle. Those things were pieces of *[censored]

 

[clinebarger]

Probably Injector Knock from 1 or more Injectors being worn out causing the injection timing to be to far advanced.

 

[Ohle_Manezzini]

Piston slap sometimes do that.

 

[The motor guy]

Oh I see.

 

[artificialist]

There may be an oil pump that is starting to fail or bearings that have significant wear.

EDIT: I don't know if the engine has hydraulic lifters or not, but failing hydraulic lifters can cause noise at idle that isn't as bad when the vehicle is accelerating.