Same issue here, 2018 Redline edition. Skip to the last paragraph for the short n' sweet summary, read on for some simplified techie background on this issue...
It's definitely an internal exhaust resonance, not a vibration noise between exhaust and anything else. Every engine (actually everything) has resonance points, called nodes. For clarification, the full term we're experiencing is called resonant frequency, and it is a natural frequency of vibration determined by the physical parameters of the vibrating object. These resonant frequencies occur all over the vehicle, hence all the dampening materials installed everywhere to isolate and change the resonant frequency points (nodes) to values outside of the normal operating environment, or to values that would provide a benefit (anyone remember "tuned port" fuel injection marketing?).
This loud droning noise that we hear is due to a resonant node of a baffle inside the exhaust (likely the muffler), and it occurs at a particular exhaust pulse wavelength which "vibrates" the baffle at it's natural frequency, which amplifies the normal exhaust sound exponentially to a point that we clearly notice a distinct change in volume/tone.
All that technical BS aside, for our vehicles the conditions under which this droning noise occurs tend to be around 2,000 rpm and with added weight to the vehicle. Gearing doesn't matter, I could reproduce this in low gears as well but generally the auto trans will shift into a higher gear so to reproduce you'd need to go into manual mode...I first noticed it when we took a family vacation trip, so 4 people, all our luggage & foodstuffs, plus a roof rack cargo container full of stuff. On the highway, with a slight incline grade, at about 65-70mph it would hit this point and drone on until I made a throttle adjustment. With the added weight and the slight road incline, the throttle position was such that the ECM/TCM (engine & trans computers) wouldn't call for an upshift into OD or a downshift for more torque. Under everyday driving conditions, i.e. not loaded down with extra cargo weight, I might hear it once in a blue moon.
Bottom line, there is nothing mechanically wrong, this noise is not a precursor to an impending breakdown. It is however a nuisance, especially on a long highway road trip with a weighted down vehicle and relatively flat road conditions... Solutions? A few come to mind, although some are just not reasonable...
- As mentioned by @Thomcat, use the manual shift mode to change into a lower or higher gear.
- Reduce the amount of weight in the vehicle (not always possible/reasonable).
- Change exhaust components with aftermarket parts. This would be a needle in a haystack thing unless an aftermarket manufacturer were to confirm their components have different resonant frequency nodes.
- Make sure your vehicle has the latest ECM/TCM firmware installed. Dealerships will do this free if vehicle is still under warranty. GM could easily fix this issue with firmware changes to shift points based on engine load (throttle position) to keep the engine/drivetrain out of this resonant frequency node. If they ever do fix this nuisance issue it will be via an ECM/TCM firmware update, so make sure you have the latest updates.
- I almost forgot this last solution, as mentioned by
@No2rdame, turn the music up really loud to drown out the drone!