OOH OOH
This hollow sound could be bad. The house we had in Atlanta had a hollow sound like that under the floor in the Living room (house was on slab) Come to find out that they had in the past a broken water line that they had fixed but the water had run that direction on toward the back yard taking dirt with it. There was no visual clue that this had happened. We drilled a hole in the slab and pumped in concrete rather than jack hammer the whole thing this worked but I was astonded at how much it took. The slab had just started to crack and we got it just in time. I hope this is not the case for you. By the way our homeowners protection plan that came with the property did not cover it. They said it was due to an impoper repair previously and therefore not covered. I have gotten that line (or improper instalation) every time I (or anyone I know) has had a plumbing issue and tried to get it fixed under home waranty.