Now that I'm in front of a computer, I can elaborate a little more than with an iPhone.
Different dispatch centers do different things, so it's hard to get specific.
In many places, they use tone and voice paging to alert responders to a call for service. Typically this is a set of tones, more often than not, a two tone sequence (though some places use a single tone). This tone group will activate pagers carried by responders. Immediately after the tones are broadcast, the dispatch center will state the nature of the emergency, location and any other relevant information for the agency.
Your scanner has a feature called "Fire Tone Out" which would allow the scanner to act just as a pager. The scanner is monitoring only the frequency for the dispatch channel and the audio opens only when the correct tone sequence is received.
What it sounds like is that you are scanning all traffic and are hearing the tones for paging.
As I noted previously, there is no setting other than avoiding (turning off) the channel that would disable the tones from being heard.