i have an idea of what this is but cant say for certain.
Does this only happen when your aircraft is disarmed? or when its armed and throttle is at 0 on the tx? is this a new build? or at least new escs?
regardless, i think it’s your escs. some escs emit a tone when they see only power and no signal, or possibly signal below a certain threshold, causing them to twitch. i think yours are doing that, but the beeps are not strong enough to be heard or are muted by rotors and frame.
my escs cause my motors to twitch and emit a tone every couple seconds when i’m disarmed. once i’m armed, my fc sends out a low throttle signal of 1050us or so(can’t remember the actual number) and that is far enough above my esc’s minimum signal to stop the beeping.
possibly calibrate your escs again, and look at any parameters having to do with the throttle low end signal.
that would be my guess without knowing more about your build.