Say Richard,

Trying to keep this more ham radio in nature, I have heard that sound I
believe you are describing on 15 meters back in the mid and late sixties.  I
often wondered what it was but you could follow the dumb thing clean up the
band.  I sure mis, tongue in cheak, the wood pecker days.  Man, I had a 4
element 20 during the peak Russian wood pecker days.  One morning, beaming
long path and working Europeans and eastern Russians that pegged the S
meter, the wood pecker got so strong, the only frequency on the entire 20
meter band, phone and CW, was about the bottom 10 to 15 KHZ.  I take that
back, I don't miss those days.

Phil.
K0NX