Do you have pending requests to become allies? I used to leave in game sounds on just for fun sometimes until they moved the ally request into the news feed area (comeon, who doesn't love that cha-ching sound!?). After they did that I started getting the random siren sound all the time since the lame game thought I had a new news item ever 30 seconds.
If you want to keep the pending requests your only option to get rid of the siren is to turn off all sounds in the settings menu.