Does anyone remember the phenomenon known as "whistlers" occurring on the HF bands? Apparently it's a "noise" that occurs in your radio receiver similar to a "whistling signal sound" and it's supposedly caused by a geological disturbance occurring in the earth's environment. I used to experience these unusual "whistling" sounds that would quickly shift in frequency on the HF bands many years ago, but not recently. Do they still occur? What do you know about them?