That’s happening because of overmodulation of the luma signal. This often happens with cable broadcasts, or when you have an amplifier on your antenna. The signal being received at your TV’s front end is too strong for the AGC to adjust, and the buzzing is the result. This can also happen at the point of origin.
You can get an attenuator for your antenna or cable feed to see if that helps. Here is a link:
http://www.hometech.com/hts/products/video/attenuators/index.html#GC-ATTEN
Plug it in line between your antenna/cable and your TV set.