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 now 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...
Turns out it was the main drain from the house, which had become blocked and was backing up. Managed to catch it just in time, the 5 foot deep drain hole was only a couple of inches from the top and both the drain at the front and the back of the house were close to overflowing. The water had risen so high that it had covered the entrance hole for the vent pipe into the system. So the bubbling was indeed caused by the lack of vent, it was just 'blocked' by the high water level due to the other blockages.