Central heating airlock?
 

On Sun, 26 Nov 2017 02:49:48 +0000
John Rumm wrote:

Probably just a pocket of air that is getting swept along as the pump
starts. Eventually you should be able to bleed it from one rad or
another.


More likely the air collects above the pump, gets pumped down when the
pump switches on (hence the swishy sound) then rises above the pump
again when it turns off - the air never reaches a radiator.