Every time I flush my toiet cistern, it makes a load groaning noise sounds like there is an airlock. This can be stopped by turning the cold tap on full in the kitchen. How do I stop this problem?? I live in a flat if this makes a difference. Many thanks

Open up the lid and check the water level. It should be 1/2" under the white overflow tube. If not set it. Now run your finger around the flat part of the flapper. If it's wavy replace it. If those all check out OK then shut the water off and take the top off the ballcock and turn the water on to flush out the supply. Reverse the washer before reinstalling. Good luck, Tom