If you open your windows and drive close to a wall/divider at 20~30mph at 1800rpm and apply the throttle, you should hear that noise pretty well. So I am pretty sure it is an exhaust leak from the down pipe to the mid pipe. I am dealing with the same issue. I have loosened up the bolts and re-tightened them, and the noise went away. It is now back. So I'll have to get the gasket, and replace it.