arghhh!! What's happening

Hello everybody, how's it going? I have this very disturbing prob with my tranny or driveshaft, between 10-15 MPH I am getting this very loud vibration coming from right under the car. Rpms are fine when this happens, engine smooth as ever, It happens with man button pressed in, with the fwd fuse in it happens. There are brand new tires on the car so no issues there, the loud heat sheild being whacked sound and shaking vibration increase with the throttle, but just between 10-15, then all is fine. I have searched through the archives and have not found anything to similar, I thought this could have been my propeller shaft but the sound did not go away with the fuse, and was exactly the same. I do not hear any odd sounds when I put it into gear, the tranny is as strong as ever. Any recomendations or opinions will be highly appreciated, thanks