Hi! It just so happens that about this time last week, I asked about an extreme "rattle", as I called it, every time I passed 50mph. Maybe it's the same problem you have.
Several replies suggested tightening the 3 speedometer mounting screws.
You have to remove the gas tank to get to them on its underside. They said it was easy to do. If you've never done this before, here's what I found on my '99 Savage.
After you remove the seat and the screws under the seat holding the tank to the frame, disconnect the rubber hose to the fuel valve and the small rubber spring-covered hose directly behind it.
Plus, loosen the speedometer connector (nut at bottom of larger spring covered tubing on the right side of the bike).
Now you can get the tank loose to get to its underside. You might hit a snag when the fuel valve wants to jam up against a rigid brass tube, but when I called a mechanic at our Suzuki dealer, he said to wrestle it any way I needed to in order to get the tank loose.
I found only 1 of the 3 screws loose, but tightening it made a noticeable difference.
I also found that putting it back together was a lot easier than getting it apart.
Hint: A second person to hold the tank while tightening the screws would have been handy.
Good luck from this "Lady Rider"
dunk