The radiator makes a strange noise when it is turned on.
If your radiator does not have an even temperature across its entire surface, or the pipes are cold on one side, or you hear a noise when they are turned on, there is probably an air accumulation 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. To eliminate this air bubble, it is therefore 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/en/support-en/irsap-consiglia-3/2096-how-to-vent-radiators, you will find a practical guide on how to vent radiators.