Help - sounds like engine brake even when clutch engaged

[Daftcow]

I have noticed over the last few months that my Mk6 Fiesta 1.6 Tdci has been making a noise when at higher speed. It only seems to do it after the car heats up and I only ever notice it when I'm on the motorway at about 60mph plus. The noise sounds a bit like the engine brake even though I'm in 5th gear at about 2000-2500rpm. The noise doesn't stop even when I press the clutch right in though if I'm still coasting at speed. It is not until I slow down that the noise stops. Anybody any ideas?