I've had a similar vibration from both my '02 and '04. In each case, the solution was the same. This may or may not help, but I found the carpeted trim panel which has the speaker grilles mounted to it was rattling against the body at the very rear edge. I took it out and applied foam weather strip (the kind used on homes with one adhesive side) to the underside of the panel (not the carpeted side) along the most rear edge. This way, when it was re-installed it presses the panel upwards against the body more tightly. I think I used 1/2" but even a little thicker would be good.
Sorry I don't have pics. One of those things I wish I had done at the time. Give it a try. I'm curious to see if this fixes it for you as well.
StingRay Bike w/cards in the spokes, Weeeeeee!