We use a rumble pak that vibrates when we grab a ball. That doesnt make much noise and gets the drivers attention (they never look at LEDs). We used the rumble pack last year to indicate when our turret sees the target. I have heard other teams use them as well (not sure who). Rumble paks are easy to reverse engineer to work with a digital out (heck if convicts can figure out how to make
tattoo guns with them, then it shouldn't be too difficult for a young engineer).
I will not say anything about safe noise levels (no one would take me seriously coming from a team that got
loud noisemakers banned 
). However, I believe audio cues should be taken from the coach only, not the control system.