Corolla / Matrix 2009-13 :: Subtle Rattling Noise When Idle Or At A Stop Light

I purchased a used 2010 Toyota Corolla LE with about 40k miles back in February of this year. Since then, the car has been issue-free and has responded accordingly.

Two weeks ago I noticed that the car when idle or at a stop light it'd make a subtle rattling noise (as if it was something behind the glove compartments rattling against metal). At 1st I thought maybe it was something inside, but as that week turned to days, I noticed that while I was outside the car and it was on and idle, I could hear the rattling from the right side of the car under the passenger seat (closer to the engine, as if right below where the glove compartment sat)--It is a constant rattle at the same frequency..meaning it doesn't rattle one way 1st and then another later. It sounds the same, but it is random.

It evolved from a subtle rattle inside to being a loud rattle outside under the car. I noticed that if it was rattling and I turned off the air, the rattling would stop, but after 20 seconds, the rattling would return just not as frequent.

It was advised to me that perhaps it was the blower...but the blower sits behind the glove compartment and should not sound so loud under the car. Not only that, but if the blower isn't on (when I turn off the air) it shouldn't sound...at least that's my logic for it.

The sound is getting louder and more frequent at this time and I'm just not sure what to do about this. I don't know what this rattling is and I've already crawled under the car and see nothing wrong, nothing shaking violently, nothing.