If the radiator is colder at top than further down, there might be air trapped inside. In this case, the radiator might also make a noise.
You can release air from the radiators like this:
Place a cup or hold a towel under the valve. Open the valve carefully with a valve key until you hear the sound of air hissing out. When water instead of air starts to come out, close the valve.
If there is no air in the radiator, you will notice that water is flowing directly from the open valve.