The negative/normal tests are very re-assuring and most likely rule-out
gastroesophagealGastroesophageal reflux disease
Gastroesophageal reflux in infants reflux disease (GERD) and laryngeal disease. Another of the more common causes of throat clearing is postnasal drip. This is often, but not always, reduced or halted by the routine use of a nasal steroid spray. You might try continuing to use the nasal steroid and add nasal washes to your regimen, twice a day. A nasal steroid spray decreases inflammation in the nose and sinuses along with mucus production. When a nasal steroid spray is used regularly it can prevent postnasal drip. However it does not provide immediate relief of symptoms. It may need to be used every day for several weeks to months to notice improvement in the symptoms. To get the most help from any nasal spray first do a nasal wash to remove mucus from the nose and sinuses. To learn more about this technique please read our nasal wash treatment information by copying and pasting this address
http://www.nationaljewish.org/disease-info/treatments/alt-ther/nasal-wash.aspx
into the address window of your internet browser. Share this information with your doctor to see if you would benefit from this daily treatment.
A third common cause is habitual throat clearing. This is a habit that may have begun with an acute nasal or sinus infection and persists, as a habit, after the initiating infection has cleared. You may benefit from consciously suppressing throat clearing, when you have the urge to do so.