anderman

I have developed a noise that is coming from the passenger side and it sounds like a playing card in the spokes of a bycicle. It is only heard when accelerating. If I take the truck out of gear while moving and the noise is gone back in gear and the noise returns. I took it to an exhaust shop and they smoke tested it and found no problems. They could also hear the noise but had no advice on what could be bad. Any have any ideas?