It sounds very much like the siphon.l which is the part of the system which when activated, allows the water to be released from the cistern into the toilet bowl. Depending on the age of the siphon, there are two options. One is to get the membrane replaced which sounds like it's torn or a new siphon.
Projects that add value to your home - Phil Spencer
Looking to extend your home but don't know where to start? Property expert and Rated People ambassador, Phil Spencer, shares some thoughts on why loft conversions are such a popular way to create more living space and add value to your home.