Pump finding in gas cent heating system part four of what is becoming a blog.?

Tried the left side of the chimney found cavity no pump though. Noise comes from chimney as though metal plates banging together-it's not the usual knocking sound as I have known that in previous houses . Ok full story:-Brit Gas chap can't find the pump of the six rad one way baxi (behind fire-concrete floor downstairs) system.Came back with supervisor, cannot find it either. Lift various floorboards no good. I pay a guy to lift laminate floor in bedroom-pump not in the nearest corners of the room. I offer to pay to have new pump fitted, but apparently it's no use as this one may be blocked and they believe the it is sucking air in to the system. Radiators all bled several times etc however one radiator seems to keep filling with air ie bathroom. No apparent signs of a leak anywhere. Is the pump liable to be in the loft ? I have offered to get a new central heating system but by all accounts the pump must be found. If no one is able to answer this question then a huge thanks for everyone that has made suggestions.