funny noise in the dash

I have a 2005 Honda Odysey and it's making a subtle but noticeable noise that seems
to be coming from behind the dash. It's hard to describe a sound. To me, I would liken
it to a sound of a boiling pot with a lid in which the lid is vibrating from the vapor
escaping. My wife seems to think it sounds like a CD player that is having a hard time
reading a disc. It occurs with or without the A/C on, I can hear it under the dash and in
the center of the dash (behind the control panel). Everything is working fine. ANy idea
what this could be?