When driving at 50mph plus I seem to get the same noise as u get when driving with your windows open from behind the dash.

I've had a look and can't seem to find anything that will make this noise,it's weird as I can hear it so clearly

Check the grommets which the wires go through, also your pollen filter just incase. Are the carpets wet?

I haven't noticed the carpets being wet.

Will the grommits be behind the bulk head(engine side) behind the heat shield?

I know theres some under the scuttle panel, not sure about anymore

some windscreen wipers can cause a noise too, plus the plastic that runs down the side of the screen - is it secure?