Originally Posted by Peterc
With limited time to catch the fan on problem, I finally heard the air blowing/sucking from the vent pipes outside the house. I later identified that this happens WITHOUT after I disconnected the thermostat from the wall. I still can not find anything that says this is a normal function. Guess I'll bite the bullet and bring in the A/C people.
With your reproducible symptom the HVAC people should be able to give you estimates, the minimum expected, and the maximum expected, cost.
If their minimum is $100 and their maximum is the same as the price of a new furnace, they have told you nothing of value. The range is too wide.
It's not like they have no idea what's causing this.
Call at least five places.
Another option is to go to an HVAC parts place and ask what part for your furnace is sold the most often, and what is sold the least often.
I did this with a Stihl leaf blower: they are not DIY-friendly and I suspected the magneto but the parts guy said they hardly ever sell any of those. BTW, it was a $3 jet that a repairman for a rental place clued me onto.