I read this sci-fI novel as a youngster. The story concerned a signal being detected, presumably from outer-space - from a race of intelligent aliens.
The main human protagonist - a young man, or older boy (if I recall correctly) - sets about to develop a better way to communicate with the aliens. And, having done so, a conversation develops. The aliens wish to interact with humans, sharing knowledge, but are fearful of what humans might do.
It turns out that the signals are actually coming from Earth, from dolphins!