Odd noise from passenger side air vent

[TucsonA/C2010]

So I am going crazy trying to find this noise coming from what sounds like the passenger side windshield air vent? It sounds like a ball rolling back and forth. So when I accelerate I hear the noise, and then when I go to stop the same noise comes back. Its like a ball rolling back and forth. Any one else experience this? And way to direct me on what to do? Thing is it seems to come and go, it will do it for a few days, then I wont notice it for a while.

[AlexN09]

Where have you looked? If it's rolling than it has to be on a horizontal surface.