What can cause constant throat clearing? I've had an endoscopy and laryngeal oscopy and all are inconclusive.

Aging rhinitis. Things to consider when endoscopy equivocal: aging rhinitis - often causes unrecognized post-nasal drip, which improves with twice daily saline sinus irrigation; acid reflux during sleep - a trial of a proton pump inhibitor is worth considering; unrecognized sleep apnea, which can exacerbate reflux.
Vocal tics. Vocal tics are common, if are combined with motor tics for more than a year it is acalled tourette syndrome, thoat clearing is one of vocal tics, usually not apparent physical cause can be found, it is related strongly to stress, it can be a cormorbidity of adhd, anxiety, and ocd, it can decrease and even disappear for a while then return back with stress, if stay constant then need to be treated.
Habit. Some time constant throat clearing can be a habit called 'tic', it can develop after uri. Anxiety or stress also can cause these kind of habits.