Steady thumping every couple seconds in the venting system

ave you had any recent work done under the hood, or with wiper cowl etc.. reason being there is a main vacuum line that goes to a check valve just left and above belt tensioner that would cause loose of main vacuum to your climate control sysem, and fords fails safe is default to defrost, let me know details or how you make out

more details for location left side by serpinttine belt follow belt back from alternator to the tensioner just left is air inlet housing for climate control system and mounted to it is two vacuum lines on goes through firewall and other comes from engine area

Automobile ventilation systems usually use a cable that connects a motor to a flap that blocks or permits air to pass to certain vents. There is also a flap which permits or blocks air from passing through the heating coil or a/c unit. A screw may have come loose cause the cable to slide without moving the vent flap, causing the flap and cable to be misaligned. Underneath or behind the dashboard is where all of this is located. Some vehicles it is easy to access and fix the cable/flap/screw problem, such a pickup trucks. Have a mechanic look at your ventilation system, as if the problem is a loose screw, it is a very easy fix, and shouldn't be too expensive with an honest mechanic.

It sounds like you have a bad spark plug. Take it to autozone and have them hook up an OBD II scanner to it for you. Its free of charge.

The problem most likely is a faulty electric blend door actuator. Diagnostics will need to be performed on the climate control system to know for certain.

no its a control problem ,make sure you have vaccume going too the heater control,!!!
and form there too the devirters!!

Blend air door may be stuck. I removed a pencil from a heater box before. It had fallen from the dash, down through the defrost vents and jamed the door.

The temperature control actuator is broken. It is in the center of the dash.

check behind the climate control its either vacuum operated or cable operated when you switch it from floor to defrost.its probably seized up or broke....

Check all connections to alt. and battery to make sure they are clean and tight, as it sounds like a loose or dirty connection someplace. Possibly something not tightened completely after battery and alt. replaced? (just enough to get the annoying light to go on occasionally)

od switch is on the shift lever. Should go off everytime you restart vehicle. Does it flash or is it on steady?