Tom and Leslie, how can I tell if a drain vent is installed wrong? Should the vent always exit from the top of the sewer drain? Mine goes down from the drain and therefore must have water inside it before it goes up the stack to the roof. There's only one vent on the toilet in a very small house.
You might be relieved to hear that the problem you're describing sounds very typical. Drain pipes don't completely fill with water. A small amount trickles through the bottom of the pipe so that air is always able to pass through it. Typically a drain will lead down to a vent, and that vent will alow air to get back into the pipe - it's more intake than exhaust. If you detect no "gurgling" sounds when flushing or using your sink or tub, the venting is probably fine. One vent in a small house, like you described, is not unusual.