Lucas Campbell

I have looked around a bit and not found anything about this, just picked up this truck at an auction, starts fine seems to run fine except when you let off the throttle quick like if you were going to shift gears the engine misses and sounds almost exactly like a Jake brake has engaged. Any ideas? I’m new to these Mack engines, I suspect a timing issue but before I dive down that rabbit hole thought maybe someone has seen this before? The truck has a ENDT 673 engine in it.