Winter camping as a kid in the boy sprouts near a lake was always full of these sounds as the ice thickened and cracked. The loudest noises were always when the sun first hit the ice, the sudden influx of energy causing the ice to expand and push against itself. A great way, if somewhat terrifying to the younger scouts, to be woken up.
Sadly, not aliens though. We checked just to be sure. And I hope, several decades on, that Keith has finally forgiven the rest of us for that.
