OH MY... how did I miss this thread!!!

AZ - I know EXACTLY what you are talking about - because I have the same noise in my xfire!!!

Humming noise coming from the rear - I notice it around 70mph or so. I thought it was potentially wind noise from the spoiler - like maybe 70mph was the resonant frequency or something.

As soon as I turn of the cruise control or let off the gas the noise stops. Step back on the gas or turn the cruise control back on... there it is.

On long trips I actually find myself humming along with it. Insanity is slowly setting on....