Zanussi ZEBF 249W frost free fridge freezer problem

Zanusi ZEBF 249 W
Type B258866
Prod. No. 925 886 660 – 01
Just under 2 years old.

When first purchased after opening and closing the fridge door, an electrical pumping noise could be heard for a short period (i.e. it appeared to be switched by the door re-closing). It appeared that the warm air was being sucked out from the fridge, and if you tried to open the door during this short period it was difficult, as it appeared you were trying to open the door against a vacuum. This behaviour has now ceased [about 2 months ago] (and very recently the thermostat failed, which I replaced) but the freezer still works, the fridge is still cold, and the fan above the cooling coils in the back of the freezer is still operational. The defrost timer seems to be working (at least it makes the mechanical whirring noise indicating the cogs are turning). Is there a pump which performs this ‘vacuum’ function on this model, and if so where could it be located, under the base of the freezer? I can not see any such pump. However, I can not believe it (the noise) was just airflow either driven by the puny fan in the freezer section or by the temperature/pressure difference between the two sections of the fridge freezer.

Any ideas? Is there such a thing in the circuit of this fridge/freezer?

Thanks in advance for any replies.