Why is coolant leaking from my car so fast?

I'm having to fill up my radiator every 40 or so miles because it leaks out so fast. I haven't checked it for leaks thoroughly, but I did look it over & i didn't see anything. But I did notice that when I start to drive after filling up the radiator I can hear the water trickling. I don't know if its just circulating or what.. Is anyone familiar with this problem? I drive a 2000 dodge ram 2wd 8 cyl