Take a long screwdriver, ratchet extension, or length of tubing & hold one end to your ear, while the other end is used as a Doctors stethoscope to probe different locations of the engine while listening for the sound to get louder. This will at least let you figure out where the noise is coming from.
Check your exhaust head pipes for any signs of rust, especially on the underside. Then use your "stethoscope" to see if the pipes are rattling. The head pipes are dual wall tubing & often times the inner tube will seperate from the outer tube & cause a rattling sound. Let's hope that this is your problem vs. something inside the engine. If it is indeed the exhaust making the noise, it is fairly easy to fix. You need to remove the pipes & drill a few holes along the very bottom (where the holes cannot be seen) of the outer pipes. Do not drill through the inner pipe. Now have a welder weld the inner pipe to the outer pipe at each hole that you drilled. Squirt a bit of paint on the welds & then re-install your exhaust system.
There is also an after-market performance header available if you'd rather go that route.
So... Find a listening device, start your bike & search for the source of the noise. Then let us know what you find out.
Good luck
J.