you more than likely have air in the system.
Do you have an air seperator such as a spiro-vent. You might have an air seperator above your expansion tank, and depending on your radiators you might have them there.
If some radiators are warm, follow the piping to the next radiator until you find a colder one, and remove the air. If you have trouble identifying the piping system call a pro.

The pump will make a little humming noise. If it resonates through out the house follow the pipe the pump is connected to and see if it is touching a floor joist. If it is put something between to absorb the sound.