Wldthng, I had the same thing happen... last week. Instead of a tree it was a filing cabinet... and it didn't shake, it was being, like, shoved... 2 inches at a time... for a total of about a foot and a bit. I just stared at it until it freaked me out so bad I kicked and knocked it over. Then nothing. And no plausible explanation for it. Never did believe in ghosts, and I still don't... for I'm trying hard not to think about it at all.