Re Goatchurch low frequency rumbles.
Assuming it was a definite external sound (and not your belly rumbling!) perhaps a heavy lorry going down the road.
Not so sure about this in Swildons' 5 - not sure of proximity to roads etc though.
I don't think I woud be surprised at any sort of noise where there is running water.
I have heard jet aircraft underground in our local mines very faint low rumble that goes on for a while and then fades - not really subliminal though, loud enough for a fairly positive id.
It would be interesting to have a seismograph set up on Mendip (or access to existing data) and try to correlate event times. A small tremor which could not be felt on the surface, or even standing up may well be picked up through your bones if you are lying flat out in a crawl as I guess CnC may have been.
Or it may be that a few feet below you in apparently solid rock small pieces of stone were flaking off and falling hundreds of feet into the master chamber that underlies all of Burrington and indeed most of Mendip. So big that it would make GB Cavern (TLCITUK) seem tiny.