I went over to a customer’s house to investigate a noise that the homeowner said was new. The gurgling noise was coming from a small bathroom downstairs that, when any other drain ran in the house, made a loud gurgling sound. The noise occurred when the toilet next to it was flushed or when any upstairs fixture was running. Since the noise was coming from the shower drain, that was an obvious place to start. The cause was also very obvious. The shower was rarely used and the trap had run dry.
It is common that when the trap has run dry, a sewer smell will enter the room but this isn’t always the case. The sound though was very loud. I simply took a small bucket of water and poured it down the drain. The sound stopped since the water in the trap now acted as a buffer.
In speaking to the homeowner, that particular shower never gets used, which is why the water in the trap evaporated, creating a void where sewer gas and noise can enter the room. I’m sure I would have been called earlier had sewer gas smells entered the room, but apparently the noise got to be enough. The homeowner now knows to pour a little water down the drain if he hears noises or smells sewer gas.