I disagree completely with the negative perspective by several posters.
Over-use is a real issue, sure, but these signalling devices are often very useful.
To say they shouldn't be used is nuts. My wife and I each have an H2YO noisemaker from
ReefNet. We are excellent buddies, but do not swim shoulder to shoulder, both of us always looking at the same items at the same time. Many times we are looking in opposite directions when one of us spots something of major interest and wants the other person's attention. The rattle is a good choice for that purpose.
If you are annoyed by one of the underwater rattles when used by someone who is not your buddy, I would ask why you are swimming so close to that person? The rattle noise does not carry very far. I will say that there is one noise maker that I personally find exceedingly annoying, especially when over-used. That is the Dive Alert signalling device which generates an obnoxious sound that carries much further than the rattle noises.