It's driving me insane... not because it is annoying or bothersome, but because I can't figure out what the hell it is.  I've heard it here, I've heard it on other people's stations (when visiting them in person, not on their transmitted signal), I've even heard it while talking to people on the phone that were MILES away and had their radio on in the background... so it isn't "local"

It's pretty random, seems to happen once every few minutes.  Occurs on various frequencies on various bands.  It does spike the S meter but it is so quick that it is difficult to tell exactly how much.  

Here is a recording.  It happens about 0:08 into the clip.

http://vocaroo.com/i/s1ylnZzxlA0q

Here's a screen cap of it on the waterfall of FLDIGI:

HF over the horizon radar, or ionosonde.

It's the noise that makes SWL interesting and frustrating all at the same time. Someone here will know I bet, just not me.

Could be anything from my experience.  Need more info.  A wide shot video from an SDR panadapter with audio would be good.

ETA:  Reminds me of my Ethernet switch and router if I get close to them with a radio.  Maybe it's your PC?

It was explained to me that the sound you hear there is a series of transmitters around the world that keep track of the MUF at all times, mainly for airlines crossing the worlds oceans etc. Linky

No panadapter here.

I have serious doubts that my computer is making noise loud enough to be heard 100 miles away, though.

I somehow missed that little detail.    Guess I was too busy listening to the noise.