Ok - so my truck has an intermittent rattle in the dashboard. I have tried to track it down - but because it is so hit-or-miss, it is nearly impossible.
I did discover one possible source for the noise - but it was temporary (and apparently a different noise, or at least not directly related to the primary noise).
My best description of the sound - it is originating somewhere between the audio system and the glove boxes. Opening the upper glove box does not change the noise. Opening the lower glove box makes it a bit louder. It sounds to me like maybe there is a screw or other fastener missing or not fully tightened/installed, so two pieces (metallic or plastic, I cannot tell) are slightly but rapidly rubbing.
The noise is not apparently related to the fan blower.
My original thought was that the noise was related to this in the corner of the upper glove box... inside is a wire harness that plugs into nothing (a filler)... What is this (or could it have been)? The reason I thought it was the source of the noise - the electrical connection inside is fairly lose, and after I fiddled with it a bit, the noise stopped...temporarily.