Rattle + sand in glove box?

[#2!]

99 SR5 V6 4x4

I've had this 4Runner for a 3 months now. An annoying rattle/knock has started up behind or under the glove box area. Only happens when while driving and a breeze is coming though the vents. Fan speed doesn't affect it. Originally thought it was coming from the tc because the sound reminded me of a old-worn clutch bearing. But I had my wife drive while I searched around for it. Looking around I realized the bottom of the glove box is lined with sand.
Everything else in the truck is spotless, quiet and smooth.

I thought a cabin filter could be worn or plugged or have a open filter cover but searching around on this site it appears the 3rd gen doesn't have one.

Any other ideas?

[kball]

Are you sure it's coming from inside and not by the firewall or the antenna?

[#2!]

As much as I can tell its coming from right behind to the lower part of the glove box. Above the blower motor.
With the antenna up or down makes no difference.

[kball]

I know you said the fan speed does not affect it but this write up might help you get in there and figure it out.

http://www.4runners.org/writeups/fan/index.html