A pocket of gas can stop circulating water reaching part of a radiator. It may leave the top cooler than the water-filled area below and can contribute to gurgling. Oxygen entering the circuit can also contribute to corrosion.
These are mechanisms to investigate, not a diagnosis from a sound or a cold patch. This lesson covers recognising the pattern and a bounded, manufacturer-approved bleed. It does not teach system draining, radiator replacement or opening a boiler. Read The Gas Boundary and Emergencies & Shutoffs before practical checks.