I've been hearing stuff like this since I started on the SW bands in the late 60s. Never thought to ask exactly what it is - just another one of those "noises" that seem to have been on the bands forever.
However Andrew2's suggestion of "Stanag 4285" seems probably right - given it sounds much like this example:
https://www.youtube.com/watch?v=u-CgZNJIAVs
Richard