The screen/cowl/headlight area is notorious for buzzes and rattles. Have a look at putting pieces of cut up inner tube between the top of the headlight and the cowling. On mine, no matter how carefully you fit the cowl, it just touches the top of the headlight.

If the buzzing is coming from below you (under the tank), particularly from 3500 - 4500 rpm, then there's a good chance its the "too-long-a-screw" issue holding the exhaust heat shield in place over the 'Y' pipe. Have a look at this thread...

http://www.xt660.com/showthread.php?t=18033

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