Where in the body produces the mucus that falls into the back of the nose, from the skull?

Your sinuses . They are in your head, behind your eyes, behind your cheek bones. Oh, it's really weird when the doc shoves his little vacuum into the ones behind the cheek bones and sucks all that crap out. All of those spaces crap is dumped into and either drains down the back of your throat, or out your nose and such.

That is produced in your sinuses. It is called "post nasal drip" I think

The entire submucosa of the nose and nasal sinus secretes mucus.