Bill,

Roel is correct. It is Morse code. I used to be a ham radio operator and we would send that before transmitting CQs and call signs. It sort of means attention I going on the air. You could look for a tall antenna all day and still not find the source. The ham may be using a longwire or dipole antenna.

Of course other sources of interference can find their way into your amp but this sounds like Morse code.

Tom