C15 Jake Popping under heavy loads or 1700 rpm?

Hey everyone. I have an '01 6NZ that started popping with the Jakes on high today. Never did it before. Only seems to do it under heavy load at or above 1700 rpm. Any ideas?

 

I haven't tried it again since it did it last Thursday. Don't want to do any further damage. Overhead was just done 8000 miles ago. (Mechanic didn't torque the locknuts)
Almost sounded like a backfire/popping sound. I have Jacob model 340B's on it, with the little cups on the bottom of the pushrods; maybe one came apart.
I'll be opening it up tomorrow to check all the settings. Bought a Cat barring tool and Injector height tool today. Having a heck of a time locating a .033" feeler gauge though. Anyone know of a source?

 

Yeah, it just started yesterday. I'm thinking something backed off for sure

 

Thanks everyone, for taking the time to reply. I've been given some great advice. I'll report my findings tomorrow

 

Ok, so I checked the overhead today and found a few out-of-adjustment intakes, exhausts and Jakes. In fact, all the Jakes were loose by approximately .003 to .004 thou.
I corrected everything and ran the engine. Tested all the Jakes via the solenoid buttons and they worked fine. I didn't find any oil leaks anywhere.
I didn't get a chance to road test it yet, but possibly tomorrow or Monday. Guess we'll know then.
Guess if it's still popping, I'll be rebuilding the Jake heads next weekend