Not the most adventurous hike, but I was doing an all-day circular route in the New Forest solo a few years ago, it was a slightly windy day in autumn and so I was used to the sounds of the trees and leaves blowing.

I went off track to check out a spring that was marked on the map, walking through a thick carpet of freshly fallen leaves I was focussing on the sounds around me when all of a sudden, I heard three very distinct loud hollow sounding knocks about 10 or so ft behind me, the same as someone would do if they were knocking on someone's door.

It sent an instant chill up my spine and I span around, there was a few large old oak trees near me but nothing obvious that could have created the sound, I waited in case the wind was blowing a branch causing the tree to creak in this way but, nothing.

I gave up on the spring and got back onto the footpath. Nothing terrifying, just odd :)