Microphones should detect the rumble. If you can do it with no other motion, then you can send morse code while wearing special exotic headphones. Or at least be Capt. Pike, with one rumble for yes and two for no. People would think it's telepathy.

But if rumbles aren't loud enough, maybe the mike needs to be down inside your ear canal.

"Earwig" headphones, with rubber stalks for the rumble-detecting silent morse-code transmissions. Eventually a bluetooth implant needed for "fake telepathy." Next, sonar beams which detect your ear rumbles from fifty feet. (Will this even work? Dunno. Maybe. Or maybe it needs laser interferometry CIA bugging tech.)