A student of mine was frustrated at the deep whistling sound of her board when she was driving. She had thought it was her board making the odd annoying sound. But it was actually the cam straps which are known for a low toned sound which builds as you drive faster.
Put several twists in the straps both on top of the board (or kayak) and in the section of cam strap between the board and the rack bar. Others have known to tie an old rag or sock to the strap which can reduces the sound as well. Or use ropes which don't whistle.