I have owned my car since new. I have recently encountered a strange problem with the fuel tank. On opening the filler cap quite a large amount of air and a small amount of fuel is sometimes noisily ejected. On occasions this is sufficient to stain the ground. I have been advised this may be occurring because I'm not doing the mileage I was, there are longer intervals between refills and the tank is more often less than half full; and consequently in hot weather a little pressure builds up. I have to say that I do not find this credible or comforting. My mileage has not abruptly changed and in any event I have tested opening the filler cap before I drive off to refill at a local garage (a few miles) and on both openings air and fuel is ejected. Could there be some sort of valve that is dirty or sticking?