What Is That Smell?

When I walk in my grandpa's house, there is a certain smell.  Spencer's grandma's house smelled like that, too.  But my other grandparents' and his other grandma's houses didn't/don't smell like that.

Today we walked into a house that is vacant and for sale, nd it smelled like my grandpa's house.  The house didn't seem dirty.  There was new paint and newer carpet.  What is that smell?  Is it mold?  Is it old piping?  Is it rot (wood, insulation, etc.)?

BTW, I LOVED the location of this house, but overall, I don't think so (and it's not just the smell).