My house is new (18 months) and about 6 months ago I noticed that out of the blue, it sounds as if someone has just flushed a toilet (I have 2). This doesn’t happen when someone has just used the toilet, it usually happens quite a while after one has been flushed. This is a single story house, if that matters. What seems to be the problem???
Sounds like the flapper is not seating right and it lets water out of tank and then refills. Take some egg dye and dump in tank let it sit there and watch if any comes in the bowl if so the flapper is bad are needs ajustment. Next time you hear it take lid off tank and see if water is down.