The day I recorded the sounds of a frozen lake was one of those wow moments where you hear something fresh (no pun intended!). The ice contracting and expanding can cause those lasery chirps. But in this case there were (many) ice skaters at the far end of the lake. Its the reflected wave of their blades impacting on the ice that you hear, much like when you a throw stone in water. High frequencies arrive first because they travel faster, creating this pitch bend effect.