I suppose this may happen with straps on normal racks too. I get massive vibration from the front straps when i have the board upside down due to turbulence. It is so bad I can't drive above 40 due to the noise, and of course possible damage all that vibration is doing to the board.
For now I'm just transporting the board tail first, right side up. This feels like it gets too much lift for the highway however and i'd rather upside down, head first when travelling at 130 110.
Anyone overcome this problem? I'm thinking of trying a towel around the straps.
Old truckies trick:
Put a twist in the strap whenever it's going more than 20cm unsupported.
Solves vibration straightaway 
Doctor Evl,
This happens with all straps on all racks.
The solution is to apply some twist to the straps, rather than trying to get them all nice and flat.
When I made my board cover I put flaps with velcro seals to go over the straps. Great for fuel efficiency and the chick dig it 
It seems that light grade nylon webbing is more prone to doing this. The heavier duty stuff does not (at least not at speeds that are possible in my wagon).
A twist may help - it did not work with my light straps over a double concave. Adding something to the strap to upset the harmonic is a good option - a towel may be a bit draggy. A board bag works, as does tieing a short piece of a heavier rope in the middle of the span.
Another technique which almost always works, is to just loosen the straps off a bit..
Or, roll with it, and stick your arm out the window to pull on the strap, and see if you can play a tune!
I just got heavier straps, and don't do them up bar-tight.
Years ago, coming out of Lancelin with my board on top I was expecting to hear that buzzing from the straps as I picked up speed in the 90k limit zone. Sure enough, as I hit about 80k it started buzzing away and I just ignored it.
When I cleared the 90k zone and tried to go faster I was surprised to find the poor old Suba was battling to get to 100k, and the buzzing was getting a more urgent sound to it,... and the temperature was getting higher,.. and the buzzing sounded a little different to ususal,.. and the rev counter was on 5500rpm,??? What the...??
I had forgotten to change into 4th and poor old Suba was snorting along in 3rd gear shouting out for help. YOU'RE HURTING MEEE. HELP!! 
D'oh. I've been careful to not put a twist in. Yeah, makes perfect sense. Add a twist or two.
Love a simple solution
