Gas leak

[Sarek]

I have an 05 kawasaki vulcan. I got it last Aug. By the time I took it for the first check up and oil change I had noticed my problem. It whistles from the gas cap. It's a high pitched whistle when pressure builds up. I asked the mechanic about it and he blew me off. And acted like I didn't know how to close the gas cap. Every time the pressure is relieved, the whistle stops, so I couldn't really show him the whistle. I've been putting gas in bikes for 20 years but I can't honestly say if I've ever had one consistantly whistle before. I don't think so.
Then in Jan. my back tire slipped in the yard and I accidentaly hit the front brake. My bike and I hit the ground and gas started pouring from the gas tank. This is my 4th time ever to go down and I know for certain none of the other bikes ever poured gas.
My question is, is this normal? Is it supposed to whistle and let the gas run out when you lay it down? It doesn't seem efficient because if it can whistle then air must be getting out and surely my expensive gas will evaperate. And if I were to have a wreck and gas poured out, well it doesn't take much imagination to think that's not safe. What should I do next?

[BuzZz]

All gas tanks have a vent to let air in to replace the gas as you use it, ussually in the cap. Most have a one-way valve so air can get in but gas can't get out, you found out why when the bike went horizontal. But if the gas gets hot, it builds a bit of pressure and your vent is leaking slightly. That's the whistle you hear and why the fuel spilled. A new cap should fix it.