When I thru hiked the Appalachian Trail I nearly went mad trying to identify that “drumming” sound. It was so deep that it felt like it was coming from inside my body, and I was concerned that it was some strange condition.

Finally, at one point I asked a group of other thru hikers if they had experienced it, and they all excitedly nodded, but none of us knew the source. Finally, some local day hiker said “oh, it’s probably just a grouse”. And the mystery was solved. Many of us were very confused for months!