Curiosity ... did you ever find out what caused the noise ... and was it fixed?Update: Tried the tone generator, and apart from making me feel nauseous, it didn't bring up any noises.
I did find a slight rattle from the driverside-door speaker, but not the right sound (as described in the OP), and was fixed by just pushing it a little.
I've got a feeling (again from my limited knowledge) that it is something within the dashboard as when I am driving it is very hard to identify the source. Almost like it is coming from the entire Dashboard rather than say a door panel.