Air is being pulled into the system and without knowing exactely how the system operates and the position of the pump and expansion it is difficult to diagnose. If the system is an open vent type (which it sounds it is) then a fitting called a Myson air ject can be be installed fairly cheaply in the system. Employ a CIOP accredited plumber to do the work if you cannot do DIY. this fitting will completly get rid of any air in the system.
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