Noise after start up

[Russ]

I have what can only be described as a ‘rushing’ sound like a kettle boiling after starting the engine on my 1.6 petrol focus. (Non Vct) You hear the pump prime on startup, start the engine then you hear the sound for maybe 2-3 seconds. I’m assuming it’s the system pressurising but it’s only happening the morning after a long run and (at the moment) it doesn’t seem to be causing any other problem. Could this be a sign of a failing petrol pump or air getting in somehow (no signs of leakage anywhere from engine or under the car)