I had this problem when I was installing my levers. Here is what I suggest: If the levers are tight to the neck and the buzzing occurs when they are engaged on the string, then try to screw in the bridge pins, a little at a time. Pluck the string after each tightening and see if it still buzzes. The string may not be snug within the lever when it is engaged and that may be the reason for the buzzing. A way to test this is to put a finger against the string between the engaged lever and the bridge pin. If this stops the buzzing, then that is your problem.
Good Luck!
Karen