Posted for "william rumburg" <lancair403@earthlink.net>:

 Kevin-
 The buzzing might be vibration of the forward counterweight extension.
Although it's not in the construction manual, you should install 'chocks' at
both ends of the extension so as to form a rigid structure, preventing flexing
and vibration.
 I believe that Rick Argente also had this same experience on his first few
flights and installation of chocks solved the problem.
 
 Bill Rumburg
 N403WR