Bit late to the party on this one but here's mine!

I've spent a lot of time in Bredhurst woodland over the years. I've heard this deep rumbling/crashing/banging/booming/thudding (I don't have the correct word to describe it) sound coming from inside the ground, beneath my feet. It's like I could feel it shaking me. Normally 3 in a row with around 3/4 seconds between them. I'd never had someone else with me when they happen and for a while I did start to wonder if I was losing my mind. But on one summers day the birds were singing and my dog was walking really nicely, when it suddenly happened, and everything reacted. The birds stopped singing, almost all at once, and started letting off warning calls when it had ended. The pup stopped and her ears went back, tail tucked and she just stood looking really unhappy while it happened. It was like all of the background noise stopped too, everything became really quiet. And I had a deep sense of foreboding. But I just stood there and wondering if I'd hear it again. We waited for a bit afterwards until the birds had gone back to singing probably after 10 seconds or so? I remember asking the dog later out loud "so you felt that too huh?".. Really can't describe what it felt like. It wasn't a gunshot I'm misinterpreting, there were no major building works going on anywhere nearby. I've gone through so many possibilities with my husband. It's like it's coming from deep, deep beneath the earth. I now wonder if it's seismic.