In all my years of being on the 11 meter, I wasn't always there, I had a long vacation from it. I remember hearing the swirling sound years ago and now, visualizing it, the sound is is like an upside down tornado. I'd get it real loud sometimes, and I'd think it was a indication that propagation might be changing. Thats probably an incorrect assumption.
But I did find an excellent article explaining the possibility of what it may be.
On the 2nd page is a posting: 27mhz ism fish hook swishers.