Many homes have what is called “makeup air” this is always on the intake side and is usually a small duct 4” or less connected to outside air in some way some pull from the attic so the air is not so cold it is rare to pull from a crawl space because that air is usually musty or damp.
I normally install a damper on the make up air ducts I install so if you run your fan 24-7 the amount of air pulled in is reduced this reduces the heating cost.
Why do we install make up air? As we started to make houses tighter we found in some cases the air inside was becoming unhealthy so a small amount is brought inside to maintain a 20.8 O2 level and keep CO2 at a trace level.
Don’t let CO2 scare you every time you exhale about 5% is CO2 your body used up about 5% O2, so with some very tight homes the CO2 levels high enough to give you headaches and other issues and the O2 levels were reduced.
A small amount of air is drawn in on the intake and the excess pressure is vented through bathroom, kitchen and if gas appliances their exhaust stacks. When your furnace is running more frequently the make up air can be reduced.
You might pull the filter and look for a opening that may have a damper or adjustable flap and close it a bit. This will reduce the amount of make up air but I know I dampen mine to 50% in the winter because I normally have the fan running in the air handler to circulate the heat in the house (wood stove).
If you close it down more than 50% watch for headache and lethargic feeling even ringing in the ears and rapid breathing can be caused by high Co2 levels but the most common issue is headaches. If you sleep with a window open you can close the make up air off the air movement will normally keeper the fresh air from the window mixed well enough.