Mysterious pumping sound continues

The mystrious pumping sound continues and still no logical explaination is forthcoming. The sound can only be heard in the library. The sound lasts not more than ten seconds. The sound is of something heavy pumping or pounding. The sound is regular. If the first sound detected is at 5:53 then the next will occur at 6:53 and so on. We have heard it every night this week that we were home. It is not heard outside. It is not heard in any other room of the house. Tonight it has been especially loud; at 5:53, 6:53 and 7:53pm. I am open to suggestions from anyone as to what or where this sound is coming from.