This adds some more controls to vehicles with sirens (like the title says):
Tapping G once will mute/unmute your siren.
Holding G will constantly play a bleeping sound (the sound that plays when you quickly activate and deactivate your siren).
Tapping Left Shift will toggle the alternative siren sound.
(NOTE: This assumes you didn’t change the key for the “Throw Grenade” and “Sprint” actions. If you did, press the key you changed it to.)