Just a note to say I still have this issue, it has never got any better or any worse.
The noise/loudness completely disappears if I press in the clutch, which suggests to me that it may be connected to the drivetrain of the car?! Who knows!!
After about 75mph the loudness change in tone from the engine is crazy to be honest, I wish I was imagining it!because I have no idea what it could be!