This issue has been discussed on another Raider Forum and the problem solved for some...after removing the tank you'll see that there are four mounting 10mm bolts under the fuel tank (two in front, two in back) that tighten steel washers against rubber cushions...make sure these are tight.
The biggest fix is to move the two large black cushions fastened with nylon ties to the frame arms so that the contact the bottom of the tank on both sides...look closely by lifting the rear of the tank and you can see if they are touching the tank, or better yet look for a spot where they might have touched the tank and cleaned off any dirt or dust after the tank is removed...they should always touch...
On mine, the right cushion was too far to the rear on the frame...moving it forward about 1/2-3/4 inch made it contact the tank...put some white grease on each pad, refasten then remove the tank, and look for grease transfer to the tank to confirm contact...tank removal makes this adjustment easier, but is not necessary to just move the cushions forward a bit.
Hope this helps you as it did for me.
Gary in Fairbanks
Edit: I might also add, as have likely others, that it's possible to temporarily lift and swing the tank to the right and set it on a pad over the seat area without removing the two fuel lines attached to the rear of the tank, providing the electrical wires to the console and fuel vent line at the front of the tank are first removed...the fuel lines are marked "don't kink" so be careful on how this goes, pay attention to any excessive kinking, and don't leave the tank in the swung position for extended periods so that they take a permanent "set".
Forgot to add that unfastening the two black plastic clamps that hold the fuel lines to the frame makes the swing easier, but in reality anymore I just close the fuel line valves and remove the whole assembly if extensive work is planned.