Perhaps I mis-described it. Have not heard it again, but thinking back, I guess it was echo-y, rather than fluttry, with somewhat of a chirpy sound as well. Gene
A few things to keep in mind. Most of the time you will hear signals from the direct (or short) path. Some RF will also head off in the other direction, and travel around the globe. Since the path is longer, it will take a few milliseconds longer to get to you. If you are hearing both signals (short and long path), it will have an echo. Does not happen often, but it not a rare occurrence either.
The ionosphere is not a perfect mirror for RF. Some RF can be reflected at one layer, some gets through to the next layer and also gets reflected. When you hear the end result, it can have echos, flutter, QSB, and a variety of other interesting things.
The more you play, the more you realize there is more interesting things to learn :-)
Tom - VE3CX