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- During/after the newscasts from the WTC, there was often an odd two-toned high pitched siren. It didn’t sound like a normal police car siren though- no emergency vehicle I have ever heard sounded anything like it. It was almost a warbling sound; I don’t even know if it was a vehicle or not. You would often hear it in the background during newscasts - what was making that noise? - MC
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Firefighters locator beacons, I think.
I’ve heard it was the firefighter locating beacons, but also I heard newscaster say it was the firetrucks/police cars’ sirens choked with so much ash it changed the sound…
Ask the firefighter.
Yep, locator beacons, worn on the turnout coat.
It’s like a transponder, so others can find you if you get lost.
It’s on a firefighter’s coat, and makes the noise when a firefighter doesn’t move for 30 seconds. You were hearing about 300 of those things going off at the same time. 
Were we? Because I strongly doubt that we were. The dead firefighters were buried under many, many feet and tons of rubble. I tend to think that the beacons we heard were the ones worn by the firefighters who dove behind cars for cover during the collapse, not the ones inside the building. I could be wrong, but that’s what I’ve assumed since the 11th.
The locator beacons (siren & strobe) are set to go off after 30 seconds of inactivity (i.e. lack of motion). I saw a video clip where the cameraman was panning the area severl minutes after one of the towers collapsed. You could see firemen sitting on curbs, slumped against cars and lying on the ground…all strobing and wailing. It was surreal. Once you realized what the sound was, it was almost too much to watch.
The “beacon” is called a PASS Device. In the case of FDNY (and my own department), it’s an Integrated PASS Device, for reasons I’ll touch upon later.
PASS stands for Personal Alert Safety System. The concept was pushed in the late 1980’s when someone realized that firefighters who become so injured they can’t help themselves generally don’t move. The PASS has 3 settings. Off, Standby, and Alert (the names vary between manufacturers, but they all have the same idea). Off is just that, off. The PASS doesn’t do anything, just kinda sits there. Standby is what the device is supposed to be turned to when you step off the fire engine. In Standby mode, the PASS monitors how often you move. If you stand still for 20 seconds or so, it sends out a chirp telling you to move. If, in another 10 seconds, you still haven’t moved (total of 30 seconds with no motion), the PASS goes into a gosh-awful loud siren. Said siren doesn’t shut off if you wiggle the PASS, it keeps going until you manually turn the PASS to Off, or the battery dies (at least 24 hours). Alert is the manual method of turning the siren on, if you know you’ve gotten yourself into trouble you can summon help yourself without waiting the 30 seconds standing still.
PASS Devices are great things, like Kinsey said, normally worn on the turnout coat. Shortly after they came out, a huge design flaw was noticed…the things only work when you remember to turn them on. I can personally attest to forgetting to turn my PASS on nearly all the time when we first got them. The manufacturers of our SCBA (the air masks) realized this, so they came out with what is called an Integrated PASS Device. Many departments (including FDNY) now use these. When you turn the tank on your air pack on, the PASS Device automatically turns itself on. The pack that FDNY uses is the Scott Air Pack fifty (its the 50th anniversary of Scott Aviation, the company that makes them). I’m not sure whether they use the 2.2 or 4.5 packs, but that doesn’t matter. The PASS, formerly a little box or cylinder worn on the jacket, is now built into the frame of the air pack. The actual sensor sits at the bottom of the pack, just behind the tank, right at the base of the back. When it chirps, you wiggle your bottom, and the pack knows you’re ok. Don’t wiggle, the pack goes into alarm. Having a false PASS activation is a sure way to piss off your fellow firefighters, as that PASS siren is the indication that there is a firefighter trapped or injured, sending your Rapid Intervention Team (group of firefighters on scene that only rescue firefighters) in to get your rear end out.
Another reason that the Integrated PASS Devices have a habit of going into alarm is because firefighters take the air pack off and forget to turn the bottle off, bleed the air out of the pack, and turn off the PASS. Quite often we’ll (well, not me, of course) just drop the air pack on the ground and walk away. Sure enough, 30 seconds later you get a reminder to shut the pack down the right way. My gut feeling is that the PASSes that were going off at WTC that the news cameras caught were air packs sitting on the ground, but thats just a SWAG on my part.
To make sure that its the same noise we’re all talking about here, I’ve got a 1.2 mb video of one of our training evolutions where the PASS device is going off in the background. You’ll hear the pre-alert chirp, followed by the full alert.