Well after testing for a while I don't think it's that. We have three toilets, on each, I flushed and then marked a line at the water level when they had filled. A few hours later I checked the water levels, and the same. Then later when we heard the noise, I checked again and all still had the same levels. It definitely sounds like a brief rush of water, as I have also noticed that if I am in the lounge and my wife briefly uses the tap in the en-suite which is directly above, you get the same/similar sound. Because it might only happen two or three times each day, it's hard to be 100% where the noise is coming from although we do have the water tank in the roofspace, the en-suite, and the boiler in the garage all roughly below each other from top to bottom, and that seems to be the area of the house that it comes from.