Most likely your Fuel Saving MDS system. It shuts off cylinders at low throttle / load situations.
Many cannot even notice it, but those mechanically 'in tune' with their vehicles often go nuts when the 'cadence' of the engine changes. It both sounds and feels as thought the ignition is dropping cylinders - which is exactly what it is doing. But in this day and age, they say that is a good thing, as you do gain a MPG or two.
Both my '07 1500 with the 5.7 did it, and now my 2016 PW with the 6.4 does it to. I was glad to be rid of it in my 2013 PW with the 5.7, though no telling what it cost me in additional fuel.
I try to ignore it, but it always catches my attention when I sense it, and it makes me crazy. Apparently if you remove the resonator or change to an aftermarket Cat-Back type (louder) exhaust, the sound is even more pronounced. Thus, I haven't whacked my resonator yet on the '16.
Most 'non car' people (my wife) has never noticed it, even when I point it out. If you are rolling along with the A/C or Heater Fan on, the radio up a little or with the windows down, it is hard to detect, and generally a non-issue.
Perhaps some Hand Held Tuners can shut it off, but I understand it operates off of oil pressure to the lifters. So, other than that, we are pretty much stuck with it.
Some can accept it, knowing what it is, and forget about it, other nuts like me just have to suffer, or just be a little heavier on the throttle and avoid coasting around - which it does very little of anyway.
*Roll up the windows, make the cab as quiet as possible and idle down a level side street. Roll into and out of the throttle. If you detect the sensation you describe, and it goes away as you roll into the throttle, be happy. All is normal.
