Has happened 3 times so far over the last 4 days. I get in and start the car like normal. Immediately a loud buzzer goes off under the dash for about 3-5 seconds. I guess it sounds like a speaker going bad, possibly the tweeter? It sounded like it was coming more from around the HUD trim plate than the tweeter though.
Does it only do it when you start the car...not just on accessories? To see if it is the tweeter, without having to pull the door panel to disconnect it, try covering it with some padding, and see if the sound changes.
finally narrowed it down to the tweeter. it happens as soon as I turn the radio on. I recently had the door panel off and something might have gotten bumped. The tweeter appears to be working normally after the buzzing.