FCS Soft Rack making a crazy vibrating noise

So i just bought an FCS single Soft Rack to transport my 9'6.
Tried it out for the first yesterday, went on really easy, strapped it down super tight according to the directions.
It's fine until I hit about 40mph, then the strap (i'm assuming) start vibrating like crazy against the roof of my car. It was so loud that I couldn't hear my radio at full volume anymore. I pull off the road and re-tightened everything but as soon as I hit the road again it started right up.
Anyone else have this issue? Thanks.
Tried it out for the first yesterday, went on really easy, strapped it down super tight according to the directions.
It's fine until I hit about 40mph, then the strap (i'm assuming) start vibrating like crazy against the roof of my car. It was so loud that I couldn't hear my radio at full volume anymore. I pull off the road and re-tightened everything but as soon as I hit the road again it started right up.
Anyone else have this issue? Thanks.