I have carried my yaks on my vehicle for many years. Twisting the straps is absolutely mandatory and I never tie down without twisting. Once in a while you need to stop and adjust your twist if you still have hum. Twisting always makes the noise go away. Sometimes I fish with friends who defate their pontoon boats and carry them in their trucks. When we leave and they don't see me anymore, they know I have had to stop to readjust my straps. At its worst it can sound like you have a helicopter following you. Twisting is the easiest answer for this problem.

God Bless,
Don