Have you only heard the rattle when in the vehicle, or have you gotten out and walked around while the rattle was audible to try and locate the area from which it is coming?
I thought I had a rattle coming from the front of the car (while sitting in it) and it ended up being that the roll hoops needed their attachment bolts tightened.
A failing Harley style compensator sounds like a tin can full of bolts being shaken about, more than an average rattle.
