The sound of water whooshing past a closing valve? Maybe try adjusting the pressure with the ball-o-fix on the supply.
Or just mutter the ancient incantation, 'that will settle down with time love' as you exit the room and don't forget to bang your toolbox on the door frame.
Next you'll be telling me that trades actually know what they're doing some of the time.
It oddly seems to fill ok now without manually adjusting the float.
There is an odd high whine in the pipes that wasn't there before though! Christ.