Ah, so your arrangement is something like this then?
View attachment 36395
If so, the pump is pulling on the vent and could be pulling in air if it runs too fast. Unfortunately, in PP mode, the pump will start fast even when it then slows down. What you have there is probably a residual of an old legacy system much more than 20 years old that worked very well with the old boilers that had negligable resistance to flow, not so well with a modern boiler. Possibly has never been fully modernised. The pump may well have been on the return once upon a time.
You can try setting down the pump as other have suggested, but my feeling is that this kind of setup is prone to this sort of problem and you might want to look at getting the system reconfigured.
One important question: Did it used to work okay and when did this change?
View attachment 36394