Have you ever been out hiking in the forest and heard a quick thump-thump-thump-thump noise? Over the years I've heard this noise on at least three different occasions, but could never explain what it was. I believe in all three incidences I was hiking along the Appalachian Trail. At first the deep sound appeared to be coming from within, as if my heart was suddenly beating very loudly. However, after a two or three seconds you realize that it's not coming from your heart, but somewhere nearby. In each incident I just assumed that it was a large bird flying overhead, but never saw any evidence of this.
Fortunately, I recently came across a discussion in Reddit which solved the mystery for me. The explanation given to the forum questioner was that it was the "drumming" of a Ruffed Grouse. A male Ruffed Grouse performs his "drumming" on a fallen log in order to announce his territory and his desire for a mate.
Here's a pretty good video that demonstrates this fairly unusual phenomenon: