Strange, I really don't know... I can't think of any standard sound effects that sound siren-ish. If you want to get to the bottom of it, you might want to check out the A-Life in the level, as Nightopians can acquire quite a few weird occupations (and build some weird things), so there's an outside chance one of them might generate a siren noise...
Oh no! It'll probably be Splash Garden's clock; the huge clock made of flowerbeds makes a chiming sound when it tolls the hour. You get a good look at it on Course 3 in the top-down section, but you fly around it on Course 2 just after passing the chip cage. I actually only noticed that the clock did this a week or so ago, so it can't be something that happens very regularly...
EDIT: On closer inspection, the clock appears to be synced to your Saturn's internal clock; it ticks away in real-time, which is why this wouldn't be an uber-regular occurence...
That'd be the car going into the garage. :) After you pass over the garage, it takes quite a while for the car to back up and finish going into it, so you'll often have moved some way away before you hear the sound.
As for the Splash Garden sound EVL, I did think it sounded pretty weird for a clock...