Could be an HFT transmitter. They are required to ID at least once an hour. I've heard one of them send a fast CW ID when they were just sending out a carrier, below the 40M ham band.
6991 kHz would be within the range that a lot of HFT transmitters seem to be operating. Most of the time I just hear their carriers. One on 6838 will send out data periodically. Haven't heard them ID with CW, but one would have to listen to 20-30 minutes of that horrendous racket to catch a CW ID, so I just don't bother.