ORIGINAL: DaveC
Here's my take. Same BH's same fletch.
I believe the noise is the fletch correcting the flight path of the arrow or the BH not flying perfectly true through the air.
The reason why-
As I was tweaking my bow I could increase or decrease the sound the arrow made while flying through the air by adjusting my center shot in and out.
If the Center shot was off the wind noise was greatly increased.
It very well could be the combination of broadhead and vanes. The vanes may be spinning the broadheads at just the right,or wrong, rpm that is generating a harmonic vibration, or a vortex "signature." All my arrows were fletched helical. I would be curious to see if they still "hummed" with blazers fletched straight.
There was no noise when shooting all the other broadheads I tried so it appears that the broadhead is the common denominator. I don't have access to my gear or I'd test my theory. [&:]