If your radiator does not have an even temperature across the entire surface, or the pipes are cold in the part, or you hear a noise when they are turned on, there is probably a buildup of air inside that, like a bubble, prevents the hot water from circulating properly causing the operating pressure of the plumbing system to drop and sometimes the boiler itself to block. Therefore, to eliminate this air bubble, it is necessary to vent the radiators. The operation is very simple and quick to perform on the Irsap website at this link, https://www.irsap.com/it/news/irsap-consiglia/2089-come-sfiatare-i-radiatori, you will find a practical guide on how to vent radiators.