Engine is intermittently resonating at low RPM

[darks1d3]

Not sure if this is "normal operation" or not but it seem like the engine resonates intermittently when cruising at constant low RPM. The speed doesn't seem to make a difference although it is more noticeable at highway speeds than lower speeds. Is this just how these trucks run or is something up?   

 

As a side note my truck does vibrate in just 2nd gear over 3k rpm. I've been told by the dealership that is normal operation though. 

[bshort]

When the DFM kicks in, I can hear it.  I have the GM performance exhaust though.  

[darks1d3]

Yeah, I'm thinking it has something to do with the DFM. The engine almost sounds like it's lugging. 

[darks1d3]

Just to add to this. I drove the truck for the first time in Auto 4WD today. The resonating/vibration was way more pronounced. It only did it for 5-10 seconds I sped up a bit and it went away. I don't think thats right at all. My 2014 Silverado never did anything like this.