Every time water runs down the sink in our bathroom a gurgling noise can be heard a few seconds after the water has disappeared. It sounds like air is coming back up the pipe. I have checked the U pipe under the sink of any blockages. The pipe goes into the wall which is plumbed into the main sewer pipe. The pipe is 38mm and goes straight down with 2 right angle bends. Could there be a blockage further down in the 38mm pipe or in the main sewer? Actually the is a hole on top of the main sewer which I can look down and there is no sign of water there to show a possible blockage.

Any ideas what could be causing this. It has only happened over the last few months after I dismantled the pipes under the sink to clean a blockage stopping the water going down.

I don't think it's a clog that's the problem. Gurgling indicates that the lavatory's attempting through its trap. This points to a blocked lavatory roof vent. Clear the vent and I bet the gurgle goes bye-bye. Good luck, Tom