This is happening on the passenger side and is really bothering me. Whenever the air is on, whether I am braking or accelerating, I hear the rattling. Whenever I am in park, even with the AC on, I don't hear it. How do I fix this?

 

You sure its the ac vent?

 

Exact same thing is happening to me! The dealer said it's possibly an obstruction somewhere near the fan back behind the vent. I don't want to pay them $70/hr to pull out some leaves or something.

 

I am 99.5% sure it is the AC Vent, but I may just have it checked out sometime soon. Yeah, someone told me it could be some brush or leaves caught in there, but I wanted to see if any of you guys had the same problem. I don't blame you for not wanting to pay that money to get something so small fixed. I will give it a couple more weeks to fix itself and if it doesn't I'll unfortunately be paying that money.