Source: Mystery RNC Guest is Clint Eastwood

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A GOP source confirms to ABC's Jonathan Karl that Clint Eastwood, the actor, director and former Republican mayor of Carmel, Calif., will make a special cameo appearance at the Republican National Convention before Mitt Romney speaks Thursday night.

Republicans have not yet officially confirmed the identity of the "mystery speaker" who will appear before Mitt Romney and Marco Rubio during primetime tonight.

In the 10 p.m. hour of tonight's Republican National Convention, a six-minute chunk of time between Rubio's speech and Romney's speech has been allotted for "applause." That's plenty of time for a standing ovation and a five-minute speech from Clint Eastwood, the respected director and long ago star of "Dirty Harry."

All week, the buzz in Tampa has been that Eastwood would make a cameo appearance, and today offered what may be an important audio clue that those expectations are correct.

During a sound-check for the night's activities, the distinctive whistling theme to "The Good the Bad and the Ugly," an Eastwood Western, was played in the convention hall.

Earlier today, Romney staffers refused to address the subject.

"Because if it was a mystery speaker, it wouldn't be a mystery anymore," said Romney adviser Russ Schriefer on a conference call with reporters.