What would cause a passenger side tapping or scratching sound at 60-70 mph?

What is your question?

There is a tapping or scratching noise on the passenger side of the truck. It sounds like a wire or a tree limb hitting the truck. I've looked and there is nothing to see under the truck. The problem starts at around 60 mph and gets worse from 65-68. It basically stops at 70 mph. If you let off the gas it starts up again by 68 mph. Please help it's driving me nuts.