Weird buzz in dash display

Okay, my first weird thing with the car. I was driving in to work this morning. I got off the expressway and slowed down to 35mph so now I'm on battery. As I turned off the main road (~ 1/2 mile from the xway) I noticed a background buzzing sound. I turned off the radio. Still buzzing. It sounds like it is coming from the dash. I think maybe it's a bug or piece of paper stuck in the vent??? I turned off the fan. Still buzzing. I turn off the headlights, the display brightens and the pitch of the buzz changes. I turn on the headlights. The display dims and the buzz goes away and does not return for the 1-2 miles to my parking space.

It is a Prius II, 300 miles on it, no mods. I found a few things on the Gen II forums for buzz, but none sounded similar to mine...

I'll be listening to see if it happens again, and talking to the dealer (I need to go in anyway, there was a small scratch in my driver's door sill that they need to touch up) but I already realize that there's more experience here than at most dealers

Thanks for any info you can offer,
Robbin