My Guess....furnace in the basement? Does it happen all the time or only
when the ac/furnace blower is running?

It's pretty much happening all the time. I do have the HRV running
most of the time. But even with that shut off I still feel draft.
The basement is about 75% underground with 8' tall cement walls. The
rear of the house is above ground and about 10 feet on either side
are also semi-above ground. It has to be coming from the HRV or dryer
vent.