The audible tones that are used to activate some fire and EMS pagers (among other things). Many of them are motorola quick call II (also shortened as QCII).
Some of the newer high-end uniden scanners can be programmed with these to keep a channel silent until you hear a set of tomes you want, like for listening to just one stations dispatches. Lots of commercial two way radios can do that as well, some can also send the tones too.