Broad Street (J/Z) station

While waiting for a northbound J/Z train at Broad St, I often here a siren or horn south of the station. Does anyone know what this is?

Does the horn sound right before the switches move?

That might be the case. It sounds before the train comes up from the layover tracks.

It comes from within the tunnel. I don't think it's intended to alert passengers on the platform since it's only audible from the south end of the station listening into the tunnel.

The T/O on the relay job used to go up and down the ladders there.. Maybe to warn them?