I've been wondering the same thing, especially considering the added cos, time and annoyance of tying in rattles to a fly.
To me, it makes some sense to add "noise" to a pattern for turbid water or night fishing, but it probably makes no difference. Consider it the density of water. It seems to me the rattles would have to be pretty loud to move enough water far enough with sound vibrations to attract fish not right next to the fly.
Pcray could speak to this concept with greater knowledge.