If you listen to footage from September 11th after the towers collapse and the dust cloud comes, you'll hear persistent chirping sounds in the distance.  These aren't fire alarms, these chirping sounds were made from something called PASS devices, (Personal Alert Safety System). 
PASS devices are worn by firefighters.  In the event that a firefighter is motionless (deceased or trapped), the PASS alarm sounds so other first-responders can find them.  On 9/11 after the towers collapsed, hundreds of PASS devices went off at the same time, and what you're hearing is hundreds of dead or trapped firefighters.  
That's what the yellow zig-zags are supposed to represent.  It's not easy to paint a sound.