When I switch the car off & get out, I can hear a hissing noise coming from behind the passenger side front wheel. It sounds like air leaking from a tyre but it isn't the tyre & it stops after a while. I also have a buzzing noise coming from the driver's side engine bay (I think) when I open the door before starting & when stationary for some seconds, then stops?? Nothing seems to be wrong. Is this normal behaviour or is something going wrong??