307 hdi 110 Turbo hisses very loud/not much power

[Jonni-pee]

Hi all,
My turbo seems to hiss very loud when i reach 3rd gear (like a noisy hairdrier!) and onwards and it seems to pull away very slowly, its not as noisy in 1st and 2nd gear and seems to pull away normally in these gears. It sounds like the turbo is working fine but there must be a major leak somewhere but as this is my first turbo diesel, and i have only had the car less than 2 weeks, i havent got a clue where to look or what to look for under the bonnet! (Dont even know where the turbo is!) Please can somebody explain to me what to look for and where to look as i have read most posts about turbo problems on all sorts of forums but im lost when it comes to measuring resistance of sensors etc like it suggests in other posts! Also, i only seem to get around 40 MPG, even on the motorway. Could the turbo fault be the cause to me using more fuel as the official figures say i should achieve in excess of 50?

Thanks in advance
John