Over the last few years I have been detecting many strange signals on the HF ham bands that I believe are not amateur in origin. I think the are the result of the digital revolution and are either various types of spread spectrum (which we aren't suppose to be bothered with using our narrowband receivers) or OTHR (Over The Horizon Radar). I know that most of these signals go without ever being noticed right in the middle of the ham bands. Take a listen and see if you have ever heard them before but didn't think nothing of them.
The orgin of these signals can probably be correlated to some of the descriptions given at the bottom of the ALE- & SELCALL-systems, tone calls, ionosondes & chirpsounders page.
aa5tb_047.mp3 - Strange pulsing sound heard only for short durations at random.
aa5tb_048.mp3 - Pulsing wideband noise often heard on 20m around 14.001 MHz.
aa5tb_049.mp3 - Very wideband (>100 kHz) chirp centered around 18.111 MHz at 0339 UTC March 15, 2000
aa5tb_050.mp3 - Pulsing signal about 30 kHz wide centered 18.093 MHz at 0346 UTC March 15, 2000