How de diddly!

Random noise... Twice now when I've started my car I've had a noise from under the rear seat. I can hear the pump prime and once the car is started there is a gurgle like there is air travelling through the pipes. Car starts on the button and it's only happened when parked with the passenger side up on the curb. Doesn't cause any other issues..

All I can think of is it's drawing air in the return pipe from somewhere..
Anyone had this before?