Why is my bath tub plug hole bubbling?

[SamHolder]

Recently (in the last few days) the drain of my bath has started to 'bubble' occasionally. This mainly seems to happen at night when we are in bed and no one or nothing is moving in the house. The bubbling is a single bubble every 5 - 10 secs for about a minute. Then it stops. Its not happened that we've noticed during the day, though did once happen when I flushed the toilet.

I have 2 bathrooms that share a wall and both baths and both sinks in the bathrooms all have drain pipes that lead to a common 'junction box' drain thing in the corner of one bathroom. I live in a basement flat (if that matters).

I suspected that it might be due to a blocked pipe, and poured drain cleaner down the plug holes of both baths. This, strangely, resulted in a definite blockage in the other bathroom. A go with the snake and some more powerful drain cleaner later and the blockages seem to have been cleared and this bath not drains much more quickly than it did before. However the 'bubbling' was still present.

What else might be the cause of the bubbling?

Is it something I should worry about? (I'm about to rent out my flat for 6 months and don't want my tenants to have a problem whilst I'm away.)

What else can I try? I thought it might be to do with vents, but I can't see anything that looks like a vent...

 

[Axel]

Partial blockage?

First thing to do is lift the cover off the main drain access chamber before the connection to the street and make sure it is not backed up.

 

[SamHolder]

spot on. only just got the time to do this now and the main exit drain at the front of the house is backed up, nearly all the way. hopefully caught in in time...

Man on the way.

Thanks!