Air in the central heating?

It could be Hydrogen forming in the system due to the wrong types of metals being in contact with each other and water.
Has anybody been below the floors to check the pipework for leaks,especially at manifolds?This is the most likely cause.Get someone to go below decks and check every pipe connection and areas of the ground for dampness..Best to do it when the system is cold.
3 years is a long time to have a problem like this

Gasman
September 2008