Noisy Central Heating Pump

[GB123]

Have a Myson Compavt CP63 Pump and the lasy couple of weeks this has started to get really noisy - sounds like a tractor whenever CH is on.
The only thing I can see to do with the pump is to potentially release some air - my own ideas as a long shot - I unscrew the central 'spindle' which comes out about 1 inch.. then a little dirty/hot water comes out... then I screw it back in again... but it's just as noisy.
Any ideas on what might be wrog with it/what to do to correct it gretaefully received. Do I just need to buy and install a new one? That an easy job? (Not done this before).

[gasman]

It sounds Terminal to me and the bearings have worn i would replace ASAP before total failure.