I rarely need to make noise underwater. When I want to point out a passing humpback whale I simply scream into my reg. (It doesn't matter what I scream, it all sounds the same underwater anyway). That works for the 10 or 20' distance my buddy is normally at.
For a louder noise, I can alway tap my tank with the boltsnap of the divelight I bring with me on every dive, or with the boltsnap on my camera wriststrap. I do try and remember to whack the tank with the boltsnap rather than the divelight or camera.
In most cases, an attentive buddy doesn't need a noisemaker to get his attention. For example, if I spot something like an octopus or frogfish that tend to disappear if I look away, I'll simply stay in position, keeping my eyes on the target, and point. An attentive buddy will come join me in a few seconds.
If you still want to get a tank banger, just ask the next time you go out on a dive charter. Then generally have lots left over by divers that forgot to take them off rental tanks.