[turbo23dog]

I think that if you are hearing a boiling sound then your cooling system is not holding pressure or air tight. Since you've been through a couple of coolant caps, this leads me to believe the cap is not the problem but likely the tank or a hose at fault and the reason the system is not holding pressure.

BTW, the pressure is what keeps the coolant from boiling. Going too far back on my chemistry lessons to explain better but temp and pressure are related...