Is this your rooster? or a neighbors? If he is young, he is just going through puberty and needs to find his voice. If he is older he may be going senile!!! Actually, have their been any changes in your neighborhood i.e. home construction, kids playing in the yards screaming, etc. This will set off a rooster as well. Our neighbor kids playing in the pool screaming at the top of their lungs seems to be a direct challenge to my rooster and he answers them accordingly. Also the train going through has really set him off lately. My rooster is only 19 weeks old so he is going through puberty and needs the whole world to know he is there.