wash-basin goes "glug-glug-glug"!!

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 !

Is this a recent thing or has it always being like that? Sounds like an air trap, sometimes the washbasin and the bath/shower takeaway joins under the floor and it can cause this gurgling -not sure how to fix, sorry.

We have a similar situation in an en-suite, off the main bedroom. It has been there for 25 years. so I would be interested also in a possible solution.

The house is 12 yrs old & problem has been with us since day 1. The outlet is on its own - (bath is a seperate 2" pipe). It sounds to me like the water leaves with such force that it takes a minute for the "pool" in the s-trap to settle but I'm not sure. It doesn't happen with any other outlet in the house though & they are all installed in the same manner - straight down from basin & direct out under the floor.

this is normal the noise you hear is the suction of the water and its beneficial as this cleans and clears out your water trap under the sink