The furnace is gas.
While the furnace was running last night, I placed my hand over the same duct. Same small amount of cold air coming in.
I'll have to snap a couple pictures when I get home tonight. I'll also have a peek outside for any vents. Would it be on the side of the house or roof? (or does it not matter?)
To make things even more confusing, I'm trying to figure out where one of my air supply branches from the supply main goes to!
In our laundry room I count 7 supply branches. One goes to the laundry room (basement), two go to main floor bathroom and front hallway, two to master bedroom (2nd floor), and one to 2nd floor bathroom. I can't figure out where the 7th goes to as all other registers/vents are fed from the other side of the house. I immediately thought of bob22's post about a duct to the attic for radon remediation, and wondered if the unaccounted-for supply duct branch goes to the attic, and the cold air is brought down from the attic in that cold air duct in the furnace room, but figured that doesn't make sense. I'll have to do a bit more investigation I guess.