Sound like the same issue as mine. Here is a vid I made with my camera in a scuba diving housing. YPC Walkon 28/10/2012 - Blue team Bridge - YouTube
That buzzing is worse in other videos I made. As soon as its out of the housing I had much clearer sound.e a mode
Does your camera have a mode where it can be activated by sound? I found the mic to be way too sensitive hence the buzzing.