Turtles communicate at very low frequencies – so low that humans more than 50 years old often can't detect it. This may help the sound travel underwater. They also really space out their calls, and sometimes only make noise a few times per hour. No wonder people didn't think they communicated before!
http://newswatch.nationalgeographic.com/2014/08/19/female-turtles-talk-to-their-hatchlings-scientists-discover/