Sounds like inadequate venting for the drain lines. Could have been build this way or maybe a clog has developed in the vent pipe. Such as birds or squrrels building a nest in the vent pipe up on the ... read more
If you don't want to loose ceiling height, you will need to remove the celing covering to gain access to the floor joist cavities.
A product we've had great sucess with is Roxul (http://www.roxul.com ... read more