Gurgling waste pipe under bathroom floor

[Glastoun]

Our basin has quite an extreme case of gurgling in the u-bend, but a second or so after that has stopped there is a gurgling/'air passing through water' noise in the waste pipe under the floor somewhere. The floor is tiled so I can't identify exactly where it's coming from, and it's more of a 'bloop, blabba-bloop' sound, as opposed to the 'gulaggle-laggle' sound from the basin. (if it helps to diagnose, I'm happy to make these noises down the phone for you).

Neither the shower nor bath seem to trigger any similar noises, and there doesn't seem to be any restriction on the basin water going down the waste, it drains as fast as the tap can fill it.

Is this likely to be a simple blockage in the pipe, or could it be something to do with the venting? Have tried soda crystals and hot water, but wondering if it's worth probing a drain unblocking tool down there.

 

[Hugh Jaleak]

Sounds like the water is filling the pipe at a certain point and causing a siphonage effect. Could put an anti vac trap on the basin but that may not cure the 'bloop blabba bloop' noise, only the 'gulaage-laggle' noise from the basin. Basin is the only appliance that does it probably because its on its own dedicated run, and possibly also the longest run, to the stack. Think the only long term solution would be to upsize the pipework to 40mm but if you cant get to it, that's not so easy....

 

[Glastoun]

Thanks. Is it worth me taking the trap off and pouring water directly down the pipe to see what happens? If it's as you say would the blooping under the floor still be there? From the sound, and the gap between the water draining away and the bloop, I get the feeling there's water sitting in the pipe, but then that wouldn't tally with the basin having no problem draining itself...

 

[Hugh Jaleak]

Possibly a dip in the pipe, causing water to collect, but as there's no physical restriction to the flow, basin will still drain as normal.