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If the straps are buzzing, you can easily silence them by putting a twist in each section of strap that is in the air (not touching the boat). It only takes half a twist. You may notice that truckers hauling very tall loads on flatbed trailers often do this as well, for the same reason. I doubt if they are concerned about the noise, but as Pat points out, a vibrating strap is more prone to wear at its contact points, so for a trucker, stopping that vibration can be a good idea.
One of my Royalex canoes recently acquired a perfect imprint of a strap on a portion of one side of the hull. You can see a perfect replica of the strap, including the weave, slightly "polished" into the surface of the hull. I know the strap was plenty tight, so I assume it must have been vibrating.