It is a Alarm (siren).
If it is a "add on" Alarm (Ford or Aftermarket)
Typically most will DisArm by the same procedure:
Disconnect Positive Battery Cable
Open Driver Door & LEAVE Open
Turn Ignition Switch to On position & LEAVE On
Reconnect Positive Battery Cable
Horn may sound for a few seconds
If not, then there should be a "VALET" Switch located inside the car
It can be the LED, on the Alarm Antenna, under the Dash, in Glove Box, etc.
If the Valet Switch is a Push Button Type:
With Battery connected
Turn Ignition Switch ON
Press & Hold Valet Switch for apprx 5 seconds to DisArm
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