click/bang noise from phantom when powering off

[Supra]

when power off the silver phantom, both from the wall socket or hold the power button for 10s+ for the stand-by mode, there's a click/bang noise come from the metal dome (by pressing the power button for 10 seconds the metal dome vibrates when there's the sound). It's quite noticeable. Does anyone notice the same noise, could it be the power amp inside the phantom, should I be worried?