Hmmf. Honestly then I really don't know. As you can see by the video, there really isn't that much vibration. I started with a wood frame then the DJI and now the flip and they've all made that noise in the video. The one thing I did move from one quad to the last are the motors. The HK 'hacker style' 1050kv motors. However they are basically the same motors that are offered in lots of kits. I have recently replaced all the bearings too with Align/abec bearings. If you listen to the part where it flies overhead you can hear it sounds pretty smooth from the outside as well. I'm OK with the noise it flies perfectly fine (50 flights?) without fail.
I use a layer of gel tape on top and on bottom of the front tab so the camera mounting hardware doesn't touch any part of the frame. Could be that.