As the last of the water drains from my bathroom wash-basin, a very annoying "glug-glug-glug" sound comes from the outlet. It sounds like the water is not leaving fast enough but the pipe is definitely not blocked. (The pipe is 2" dia., drops straight down & runs out underneath the floor.) The sound lasts only about 10 secs. & only at the end of the flow. I can live with it but its driving "she who must be obeyed" crazy !