Quick high-pitched double chirp

My iPhone 6 has been emitting this extremely high-pitched double chirping sound for the past few months. It’s not related to any particular app and there is no error message or anything visible associated with the sound. It can happen three or four times about a minute apart and then stop until the next day. It went away for a month or so and recently resumed. I’m totally stumped. I keep the operating system updated and am currently running 12.2.

I’ve seen one other mention of this in these forums, but no other mentions of it on Google.