Sean Hoare, a whisleblower who claimed both the Sun and News of the World engaged in various phone intercept practices to gain stories, died in unexplained circumstances at a house in Watford at 10:40am.
The police are not treating his death as suspicious.
Hoare had spoken to the BBC, Guardian and US newspaper the New York Times over the past year to claim that phone hacking was endemic at the tabloid and that the editors were fully aware of the practice.
He said Coulson, formerly a friend, had actively encouraged him to hack phones.
Hoare was sacked from the News of the World over drink and drug problems.
Coulson denied all phone hacking allegations.