~4800 khz propagation tests?
 

I hear these from time to time, but this time I get it from around 4750 khz
to past 4800 khz. A "klaxon" type of sound every 0.8 seconds or so,
repeatedly for hours. Also on other nearby frequencies, sometimes a
carrier appears/disappears at a different rate, like every 1.5 seconds, but
there is no sound. What is this? Propagation testing?