Induction unit makes buzzing sound
As you probably know, induction burners produce an alternating magnetic field that induces a current in the cookware. This process can produce attractive and repulsive forces as well that cause vibrations.

The problem can be caused not only by poor sandwiched construction but by an uneven bottom surface, or loose parts or lids.

I have read that some people have successfully used silicon baking pads to dampen the sound.

This page has some useful information on the subject:

http://theinductionsite.com/proandcon.shtml