Some more: Nokia wired headset with four-pole plug (came with C7, no idea what type) - strong tapping sound in left channel; audio plays without tapping sound. If answer button is pressed during audio playback, volume increases a lot.

Another Nokia contraption (sorry, no idea what type, I've got closets full of these, having had dozens of Nokia phones since whenever) with media control buttons + answer button and a 3.5 mm socket - with Koss PortaPro headphones plugged into the socket, same tapping sound, the media buttons (play/pause, stop, forward, backward) do nothing.

A novelty handset with four-pole 3.5 mm plug and a polarity switch: if the switch is pushed in, no tapping sound. Haven't tried calling with this thing yet. This works perfectly with my C7 if the switch is out, and the answer button answers and hangs up. IOW something seems to be wired differently than in Nokia phones.