I think you're hearing the seat belt buzzer. Try buckling (and yes, close the doors too) prior to engine startup and see if it still makes the sound.
If the sound does go away after do you that first, you'll know for sure. If it doesn't stop, give us some more details. The early 5 or so tones due to not being buckled can not be overridden. The continued warning tones for not being buckled can typically be overridden with TechStream at your dealer or maybe with Carista. I have the continued alarm overridden on both of my Toyota's.