Probably due to an airlock in the cold water system (feed pipe).
Probably an air-lock somewhere !
It's the water going through the pipes.
A toilet flushing that creates a groaning sound in the pipes of a house is usually caused by gas buildup in the lines. As the waste and water pass through, it displaces the gases causing the audible noise.
drain must be freezing some where, when water is added it thaws and may cause gurgling,
Central Heating Power Flushing can be necessary if there is a large build of of substances within the central heating pipes, the boiler, or the radiators. These substances can cause build of of pressure within the boiler to dangerously high levels, as well as preventing hot water from flowing properly through pipes.
Replace the fill valve or throttle down the isolation valve
In order to clear a blocked water supply line, simply clear the blockage by flushing the pipes. To flush the pipes, pour some drain cleaner into the affected area and then rinse out with hose water.
My heart sank as the last of the water went gurgling down the drain.
"Gurgling" can function as a verb or a noun, depending on how it is used in a sentence.
making or characterized by a hollow bubbling sound."a faint gurgling noise"(of a baby) making a contented sound."a gurgling baby who will one day inherit the throne"
Probably nothing wrong with the toilet. It is more likely that your pipes are full of rust or mineral deposits.
The presence of fluid and/or mucus, giving characteristic signs when listened with a stethoscope e.g. rales.