I'm not a doctor, so take this for what it's worth.

You may have post-nasal drip, in which fluid from your nose and sinuses
is dripping down the back of your throat.  This is due to some kind of
rhinitis, inflammation of the nasal passages.  There are several possible
causes, including chronic infection and allergies.  There's also 'idiopathic
rhinitis', meaning that you've got it, but for no particular identifiable
cause.  It's pretty common, especially in people who have sinus problems.

I suggest you see the ENT again, or another doctor.  They may be able to
help you figure out what the cause is, and offer treatments.  One that
works pretty well for idiopathic rhinitis is an inhaled corticosteroid
powder.  They come in little dosing dispensers which make them very easy
to use.

If you want to try things on your own, you could try an antihistamine.
If it clears things up, you probably have allergic rhinitis, but then
you have to figure out what to do next.