Doesn't really make much sense how the zombies just "suddenly" show up whenever you activate the signal, and a tornado siren playing off might seem a bit cliché.

However, a Flare Gun is a pretty common device used for "Extractions" in general. And so, what if starting the extraction event that summons the helicopter has you fire a flare up in the air, which lights the area where you'll be able to be extracted from? This can even give the zombies a realistic reason why they're approaching you suddenly, even an hour or two -before- the helicopter arrives.

In addition, the light gives the player(s) a visual on where they need to stay close to during the conflict.