I was out for a walk last night. 9pm, Derry Northern Ireland, -2C and snow on the ground.

I was enjoying the silence and keeping an eye on the fields for nocturnal animals when I heard a ticking sound from just beyond the road. I walked across the verge, stood still and it came again. I thought it may be a roosting bird or amphibian, but there was nothing except snow and rushes in the field. As I waited, a second one started a few metres to my left. To cut a long story short, I eventually pinned it down to the wooden fence posts. I wiped off the snow and the ticking continued - not constantly, but intermittently. I shone a torch to see if there were bubbles being squeezed out of the wood, but found none. I warmed the ice with my palm and that didn't stop the noise. There was no electric fence nearby. The wind was negligible (powdery snow was adhering to the strands of barbed wire).

Pressing my ear to the timber (luckily no one came by) I could hear a distinct ticking at approx 5Hz and - sometimes - a deeper knocking sound at a slightly lower frequency. I've put this question in "animals" because I suspect it was an insect of some sort. Any takers?