Have soft racks too. Used same set for past 3 1/2 years so far for as many as three boards at a time. One strap wore out and broke, so just tied back together. Buzz can be annoying. Have twisted straps, and that helps some. Driving under 50 also helps. Driving into the wind can add to the buzz factor. But the BIGGEST issue for me is when it rains. Straps act as wicks, and water drips in car. Also hardware on racks will easily scratch paint on roof if you're not careful.
I wondered about that being an issue... Another thing I have to work around is that my Jeep has the radio antenna right in the middle of the roof in front of the back lift gate...
When I had to use surf racks- the old school kind where my favorite. These- super easy to attach a board - just a bungee cord and hook-and it holds better then soft racks, even at high speeds: http://www.rackyourboard.com/products/huntington-surfer-surfboard-roof-rack-1 If i was serious about transporting a board- and didn't want to put it in my car/truck- I would get something better then soft racks. I would only use soft racks for car rentals during surf trips