Bleed Pump
Pumps can some times get air trapped in them when you've worked on the heating system. This will cause the system to be noisey and can also lead to complications, so you should bleed the air out of the pump. Please allow to cool before doing any work and turn off the power to the pump.
Turn off the power to the heating system and check there is no power at the pump with an electrical tester.
Place a rag or a suitable container under the pump to catch any drips of water.
Undo the bleed screw slightly (but do not take out completely) on the front of the pump by turning it anti-clockwise.
Once the hissing/gurgling has stopped and the water is constantly dripping (with no noise present) tighten the bleed screw back up by turning it clockwise.
Turn the heating back on and check that the air is gone, if there is still air in the pump repeat the above steps until clear.
Job Done!
This is the bleed screw, located in the centre on the front of your pump.