Could be mud daubers or other critters in the air flow path, getting sucked through the fan blades. It doesn't have to be anything large to make a rattle - a hard shelled beetle will do it. Could also be the fan itself, which is typically a squirrel cage type with plastic fins that have been known to disintegrate after a few years (this I know from experience!). The sound of plastic shards flying around inside is startling, to say the least.
Former owner of 2004 American Tradition and several other RVs
Home is in the Ocala Nat'l Forest near Ocala, FL
Summers in Black Mountain, NC