Not the same car, but try this.
I had the same problem with my Isuzu - when running AC/Heat, sometimes condensation builds up in the system. If it was below freezing the day you had this issue, the condensation can ice the fan blades causing the out of balance sound. What you do is run the heat at full blast until the fan blades would thaw - noise gone.
Might want to try that... Just a thought.