Harmonics...lots of them and their secondary/tertiary/etc (resonances) and
EVERYTHING contributes...even the distance between pavement tar strips vs the
speed traveling
Easiest is to change speed and normally slowing down to see if that reduces and/or
changes where the harmonic starts
Once a harmonic is reached...the other components/systems/etc then play with their
resonances and if sync'd...building and building to ever higher amplitudes
The softer the lower the harmonic point...the stiffer the higher the harmonic point
Meaning a softer suspension (ride quality, lowest class tires, aired down, soft
suspension, soft shocks, etc, etc) will have a lower frequency harmonic point
Then the moment (lever arm) between the TV wheel base vs the trailer (contact
point with the TV to the trailers axis (axle center line) of rotation
Too many things to mention and check...why said to change speed as the quickest
and easiest test to find that harmonic point.
Next easiest things that cost more money and time would be shocks, tire PSI, class
of tire change (going from P class to LT class), etc. Within that is the WD
Hitch spring rate & ability to WD weight around. Even changing the tongue weight
changes the harmonics
Good luck
