Strange 'click/tak' sound in right earphone in KZ Castor Pro

I brought the Audiocular x KZ Castor Pro X type C from Audiostore. Everything is good including the sound, however I have realised that there is a stange sound like a click or 'tak' whenever I touch the right earphone and also when I move my head the sound appears. It is there even it is not connected to the wires. Any idea what it could be? I have already connected with Audio Store and they have asked me to send it for checking, but I just wanted to know if it is something normal in iems or not.