There can be many reasons for mucous coming out of throat. One of the most common cause is post nasal drip where secretions from nose drip into the throat causing discomfort and frequent throat clearing. To diagnose this condition you may need to undergo nasal endoscopy and CT SCAN if required to identify exact source of secretion. Other cause can be reflux laryngopharyngitis . if you have any symptoms such as recurrent heart burns, burning sensation in throat, dry cough then you need to be evaluated for this condition