"They told me he has been released, and I'm waiting for a report," the source said on Thursday.
The British embassy in Baghdad, which has been following the affair closely, was unable to comment immediately on the report.
Also on Thursday, Saadoun Janabi, a lawyer for a former Iraqi judge who is standing trial along with Saddam Hussein, was kidnapped, a senior legal source involved in the trial and police said.
Police earlier named him as Saadoun Dulaimi but other police sources also said his family name was Janabi.
The legal source said he was a lawyer for Awad al-Bander, who sat next to Saddam in the dock on the first day of the trial on Wednesday.
A resident of Baghdad's Sadr City neighbourhood told The Associated Press that he was sitting next to the 33-year-old Carroll after his release.
The kidnapped lawyer represents
al-Sadun (L), a co-defendant
The resident spoke on condition of anonymity because he did not want his involvement in the release to be known.
The resident said Caroll was kidnapped by criminals and that a group of Sadr City residents raided the area in which he was held and freed him.
His account of the release could not be confirmed.
Carroll was on assignment for The Guardian newspaper when he was kidnapped on Wednesday in Baghdad.
According to security sources, the unidentified abductors arrived aboard two vehicles and forcefully took Janabi from his office at 8.20pm (1720 GMT), according to security sources.
Janabi is a lawyer for Awad Hamad al-Bandar al-Sadun, one of Saddam's seven co-defendants in the case.
Al-Sadun is a former chief judge of the revolutionary court and deputy head of Saddam's office. He is charged with sentencing 143 Dujail residents to death following the 1982 attempt on Saddam's life.
He wore a traditional Arab red checkered scarf on his head as he sat next to Saddam in the front row when the trial opened on Wednesday.