singing tie-down straps...?

I use an alligator strap to secure mt kayak to my car roof …but when the car starts going fast they/it starts humming and resonating loudly.

what can I do to stop this sound wave vibration from happening…?

Rope instead
I use to have the same vibrations/hum while using webbing style straps that are very popular and a canoe on top. I switched to a good quality rope, and presto, no more noise! I don’t know if it is the round profile or what, but it made a huge difference for me.

whats an alligator strap?
Dont know but if its webbing two methods may help.

One is to put some twists in it.

Another is to crimp it periodically with twist ties.

I make
acouple of turns around the cross bar with mine to keep from having a long stretch of web which does sing and loudly on my car top

Agree with twists
otherwise flat webbing makes a fine flapping wing.

Yes, twist

turn up
the radio till ya can’t hear it!

Drive faster

To achieve the best twist
I wind the free end of the webbing a few times around the taught end, then pass it through the open door into the car. It will stay put once you close the door on it. This all takes about 15 seconds to do.

If it vibrates, I just pull a little on the webbing, causing the taught end to unwind some until the “sweet-spot” is reached where the noise stops.

Twist - not just for noise
That high speed flapping/resonance will stress the heck out of the fibers in the webbing at the ends of the section vibrating (usually a flat hard point on the rack/saddle that focuses the energy). After a while, they’ll tear like postage stamps.

That, physical damage (cuts, abrasions,etc.), and UV degradation are the primary causes of strap failure.

TWIST! (and inspect regularly).