Hi. I Just bought a 2000 1500 Intruder LC, and I noticed the same type of noise 2 or 3 times, but very briefly. I think it may be a baffle rattling around in the exhaust resonator under the gas tank, or maybe the bolts holding it in are loose. From the right side of your bike, look past your exhaust pipes and you will notice they are diverted through a black metal box below the gas tank (resonator), behind the engine, and then back out. This resonator is what apparently gives this bike it´s nice exhaust note. I notice that at times, the bike will backfire (a small "pop") on a downshift. This is noted in a couple of test drive articles on the bike. Maybe this is loosening up the resonator or it´s guts. I hope this helps. Happy motoring. Gord.