It's quite natural for amphibians to croak, especially at night. My White's Tree frog does this almost every night between 12:00 a.m. to 6:30 a.m. I don't find it bothersome, nor does my sister who shares my room. It is quite possible that he is trying to attract the female with his croaking. Or it could be that the barometric pressure over the past few days has had an effect on him. When there is heavy rainfall or snowfall or thunderstorms, my Whites Tree Frog tends to croak more than normal. If it's the weather that's causing him to croak, there's really nothing you could do. You could try removing the female for a few days and see what happens, but what Terry says makes perfect sense (I have observed this behaviour in male Wood frogs). If you haven't changed his temperature, humidity levels, or lighting schedule over the past few days than these might not be the issue.