The former spin doctor wanted to check the phone records of NotW reporter Rav Singh, who was suspected of leaking information to the television celebrity, Timothy Langdale QC, defence counsel for Coulson, said.
The jury in the phone hacking trial was shown an email sent by NotW reporter Chris Tate to his then news editor Ian Edmondson on May 20, 2006, about a story involving Calum Best, the son of footballer George Best, and Lorna Hogan, who claimed to be having Calum Best's baby.
"Had a call from Lorna Hogan last night claiming Calum Best had texted her at approximately 10.20 saying 'how can you be so low selling picture of unborn child?"' he wrote.
"She says he must have been alerted from within News of the World and that she had a similar text before our other story on pregnancy.
"She believes he is being tipped off in advance and freaked out last night threatening to go to Max Clifford today and that she should have stayed with him instead or returning to us. Doubt he'd be best pleased by her. Flakey (sic) as you know."
Hours after the email was sent, Coulson sent a message to Edmondson asking, "you think Calum a leak?"
Edmondson replied claiming Mr Best had been "bragging" about having "close mates inside News of the World", the court heard. Coulson then sent an email which stated "do his phone", the jury was told.
The trial was adjourned and the jury will return to court on Tuesday.