I was at a friend's house around 12 years ago, we were in the basement watching tv, when his mom's boyfriend comes down the stairs and tells us to keep it down. We looked at each other, confused because we were literally just watching tv and it wasn't on high volume at all. We said we weren't being loud, and he said 'You aren't yelling?' and we shook our heads. He just sort of does the 'huh..' look and tells us he and my friend's mom had heard someone screaming coming up from the furnace vents, so he though it was coming from the basement where the furnace is. We didn't hear anything and were like 15 feet away from the furnace.
Sadly I can't verify the next story because I wasn't there, but my friend (from the basement) and another friend say they were upstairs in the kitchen one night hanging out, and they heard a scream, clear as day, coming out of one of the furnace vents. Freaked them out, naturally, but I never did get to hear it.
Probably just metal creaking, but it was weird that we were so close to it and didn't hear anything that could be interpreted as a scream.