Slow consistent knock under floor of house
I have a slow, dull repetitive knock under (I think) the ground floor of the house. The floor is solid concrete. It is equally intermittent in time - about 5 seconds in between each knock and is not loud. The knock sounds like somebody gently tapping a concrete paving slab with a lump hammer at a considerable distance. I have checked the exterior of the house and can find nothing knocking the brickwork in the wind. The knock is much less pronounced upstairs - in fact it is barely audible in most upstairs rooms. We have recently had a conservatory built with a new radiator installed.