Redbow
Senior Member
Squirrels jumping out of trees when startled make a noise similar to that !
I think many times deer will hold fast if they think they are hidden...they may stomp once, but if you don't make eye contact with them, and you continue in any direction that is away from them, they won't move, particularly if they haven't winded you..I've heard the sound your describing as well and this is the only thing I can figure....
I've seen squirrels fall from 40-50 feet, they spread out their legs and land on their stomachs. It makes a huge thud when they hit the ground.