I second this. Try filling all drains and pipes with water. Grab pitchers, fill them up, and empty them down the drains, you may need to do this every few weeks as the water evaporates out of the pipes.

If for some reason you only have straight pipes down to a source you can't get to, try installing P traps if you can so water keeps the air from the sewers from coming up your pipes/drains and into your bathroom.