Tapping noise on Defrost setting and CEL coming on

I have a 2002 Ford Windstar SEL, that makes a quiet tapping noise, that sounds like it's coming from the engine (as opposed to the heater), when I select the defrost setting on the heater controls...

It gets faster with the speed of the engine, so if I am stationary and rev the engine, the tapping gets faster and slower depending on the revs. But then disappears completely if I select either the feet or vents setting on the heater controls.

I have also just started getting the check engine light coming on with the codes P0171 and P0174, at the same time as noticing this sound, but am not sure if it's related at all...

Any ideas anyone???

Thanks for any help in advance!

The actuator for the diverter for where the air goes.....Defrost, Floor, vents....... is vaccum driven.
It sounds like you have a vaccum leak that is in this circuit.
Defrost is the setting that it will go to if you loose vaccum to the system.
You may have a damaged vaccum line.....in this (airflow select) loop......
I would recommend that you follow the vaccum lines for this all the way back to the upper intake manifold.
My '96 has a "box" mounted on the passenger side inner fender that is a resevoir.......holds vaccum when the motor is shut off.......thereby holding the diverter door in the selected position until the motor is started again.

The lean codes could be related to this problem, as the most common cause of the lean codes is a vaccum leak.
All those vaccum lines connect on the back side of the upper intake manifold.....where it is difficult to see them.

Consider getting a smoke test done 1st, befor throwing parts @ it!
Any unmetered air leaks can throw lean codes.

Do a Google ''Youtube Auto smoke test''