What does Black Mold smell like?

We just had a sink overflow in our house a few days ago and now the entire basement smells like rotting paper and fabric. The thing is though, we have searched for any wet paper and fabric and found nothing, or rather what we did find we threw away. But now, when ever our furnace kicks on, we can smell the rotting smell again. My fear is it may be rotten dry wall and rotten dry wall means a good chance for Black Mold. Our house is about 80+years old, so that doesn’t help the matter any more, not to mention it is very bad condition anyway.
I’m just wondering if Black Mold has a smell, and if it does, what does it smell like? I don’t want to ripp out all of my ceiling and walls to find out that it was just a wet towel from years ago in the wall.
(And yes, I have found old towels in my walls before, and even an old hammer from the early 30’s. That is how crappy my house is.)