andy
James' sarcastic remark about a mouse was correct, look deeper inside the air box and under the dash, you will most likely find nesting material padded with fluff from your air filter.
Park your car with the windows rolled up and the blue vent lever closed, this will help keep the critters out of the air box.
Once a mouse has found a path into your car, they leave behind a scent that others will follow after you have removed the current residents it is not uncommon for another to move right back in. On farm trucks and vehicles I have had to install metal screens asross vent openings to keep the varmits out.