Wayne Harrison wrote:

i have tried "twirling" the straps, using various tensions on the tie

downs--and nothing works. above 40 mph, this ceaseless, loud droning
noise
permeates the car's interior.

surely there is some simple solution.

Wayne, try adding a vibration damper to the straps. Something like an
old tenis ball with a slit on two sides. Run the webbing strap through
that. Or, more accurately, those...probably two per strap, one for each
side of the boat where the straps aren't touching anything and can
thrum at hughway speeds.

Even a chunk of ethafoam would probably work, just something to dampen
the straps ability to vibrate.