Are your radiators giving off a bad smell of eggs or similar when you bleed them of air? (the removal of air via the air vent) Then this could probably be a build-up of a chemical called Hydrogen Sulphide in the water. Hydrogen Sulphide which can build up in central heating systems not only causes a bad smell but will cause corrosion sludge and magnetite within your central heating system over time and will require removing. A few well known brands of central heating cleaner

The harder the water, the more minerals it contains and the more likely limescale is to build up.