This fellow was bellowing, a very low, very loud rumble that is used to claim territory and attract a mate. I did not manage to catch it in this photo, but when they are making the rumble, the water around them vibrates up and down.

During the right time of year, male alligators will often bellow in response to any low, loud sound. I have heard them bellow at helicopters, low flying planes, and even garbage trucks.