Oh radiators, those overworked, underappreciated winter necessities. Who ever really notices them...? Unless they start to gurgle and groan, and make all sorts of grotesque noises.
As an integral part of your home's efficient functioning, they periodically need attention and care, too.
If you can hear water moving through your radiators, there is air trapped inside your pipes, reducing the efficiency of heat delivery to your home. It is time to "bleed" your radiators to let the air escape, leaving a closed circuit of hot water only.