I have read all the posts in this thread so far and would like to add my 2 cents worth.
As an ex National Bureau of Standards Lab Rat that specialized in Physical Dimensional measurement, calibration and repair and who is currently dealing with this issue on my new 2019 RT with just over 2k miles on it I, from experience, know it is belt tension as a function of length between pulley contact points.
What's occurring is Resonant Frequency Amplification. There is tons of math and physics involved, so I will try to explain it simply. I will be the first to admit I hated doing resonance work in the lab, 'cause it was too much trial and error because of all the variables...but it was part of my job. To give you an idea of what they were like, we used to call them "Guess and Go's".
Simply put, Resonant Frequency (RF) on our drive belt is primarily a function of length, temperature and tension. Length remains a constant determined by axle adjustment (Distance between pulleys). Tension is constantly variable through load, speed and belt temperature. As we ride, the belt is always "Twanging" like a guitar string, whether we can see it or not, and the frequency of the "note" is always changing. However, once the note hits the resonant frequency of the bike's frame, that note amplifies and is felt throughout the entire machine as a vibration as the bike "rings" in harmony with the belt, which in turn, causes the belt to ring even louder. It turns into a self amplifying circle. Any change to one of the variables causes the note to rise above or drop below resonance causing the vibration to diminish or stop entirely. That's why it will come and go even on cruise on level ground.
A shifting breeze will even affect it because of the slight change in loading.
Anyway, all we can do is find the happy medium to mitigate the effects of it as it will always be lurking there somewhere, at some speed, at some load and temperature.
Using a tensioner is one solution, as it effectively shifts the frequency into a higher octave by "halving" the belt length between contact points, much like placing your finger on a guitar fret halfway down the neck. Changing belt tension is another means of shifting the resonance, like tuning that same string.
I'm looking to shift mine first through belt tension so the RF occurs at above 80 since I rarely ride faster than that. Failing that, I will look to adding a tensioner and work with a combination of the two to find the "Sweet Spot" for me.
Currently, my vibration at 78~80 is so bad it almost feels like I'm riding a vibrator, and it's driving me nuts since I do 50 miles daily freeway running at those speeds.
