Coolant overflow tube

Why does my coolant overflow tube always make condensation? I took a quart jug and put on the end of my overflow tube and in about a week it will have a noticeable amount of water in it (not coolant) just condensation from the tube. I'm not losing any coolant. Is this normal? Also when I shut the truck off and put the end of the overflow tube up to my ear I can hear a little air coming from the hose. So as I did this I turned the radiator cap about an 1/8 turn or so while I was listening and the air got worse turned it back tight and the air noise just about went a way. This just seems strange. It almost seems like the radiator cap will not seal. I have also put a gauge on the radiator and monitored the PSI while driving and it never gets above 7-8psi. Truck has 10psi cap.