Peugeot 307 sw hdi 110 - Turbo seems too noisy?

My 307 sw turbo sounds like a noisy hairdrier when it kicks in! There seems to be the initial boost but if you keep your foot down, it starts getting louder and ther doesn`t seem to be much power in high rev range! Are all 307 turbo`s like this or is it faulty? Sounds like a very loud hiss in the cabin all the time and its very off putting whilst driving on long journeys! Also, i only seem to average about 40mpg, even on the motorway. Whatcar figures say i should get in excess of 50 combined, could this point to a fault with the turbo?
Any help is appreciated!