OK I think it has to do with build quality and how well the usb ports are sealed, I got all my friends to try this and none of them with iphones heard it and same with those that had q10s.. but those with z10s all hear it and it's very faint on my gfs android (nowhere near as loud as my z) so my conclusion is all phones do it (micro relays) but it's matter of how well ports are sealed to mask the sound

rivateye: