I have just developed the same problem on my S60, which is 3 years old and has done just 24,000kms. The whine, which comes and goes, is very loud when outside the car and is only heard when the tank is below half full and the engine is running. When I open the fuel tank cap, there is heavy gas escape and I can hear the fuel inside bubbling vigorously. Problem seems worse in hot weather. My dealer says he hasn't heard of the issue before and seemed surprised when I told him there is discussion on the Volvo Forum about it. Just a bit alarmed that it might explode.