We purchased a house 3 months ago that is about 50 years old. Northern Ontario. There is an audible flush coming from inside the wall from the upper level right to the lower level and you can hear the water going into the sewer pipe. My carpenter brother in law checked into it a bit this weekend. There is no water running sound for refill, you hear a trap release, a sizable flush, then you can here a trap or something swing shut. There is nothing in the roof/ceiling of the upper level i.e. cisturn or something like that. Nothing visable anywhere. Does anyone have any ideas what this could be before I start tearing walls out to figure it out?

My brother in law has never heard such a thing before and he is stumpped so it's not me!!