This signal went off-air at 0245 UTC. It might have been the oddity station known as XM / Whales / Backwards Music. However normally this station does not have such well defined audio edges. The signal was 2.8 kHz wide, with a sharp bottom edge at 300 Hz (assuming 6945 kHz as tuned frequency), this is pretty typical of communications SSB transmitters. So it could have been someone wanting to sound like XM.
Tentatively, and because 6945 kHz USB is a known US Navy / USCG frequency and XM is suspected of sometimes being US Navy in origin, I would say probably XM or related phenomena.
T!