C15 Jake Popping under heavy loads or 1700 rpm?

Hello, I know this post is a couple years old but I'm having a similar problem with my 03 6NZ. If I used the jake when descending a hill I would get a bunch of popping, back firing type of sounds. I also get the same sound, but no where near as loud, when just driving at highway speed. Just this intermittent popping, detonation, stumble...call it what you like. So I took the truck to the Pete Dealer and they rebuilt the jakes, replaced the solenoids and adjusted the valves. Now it sounds like it should with the jake on but still makes the same popping, backfire sound when driving down the road without the jake. How were you able to resolve your issues? Thank you