Could be turbo. Is the noise still wurring after the engine has stopped or does it stop when the engine stops? The only thing still spinning after the engine stops is the turbo.
check for air leaks in the servo pipe and vacuum pipes. A split that sucks in air after the engine stops could give you the same type of noise.
If you have a turbo issue, check the crankcase breather isn't blocked by pulling out the dipstick with the engine running, no oil should come out, if it does then it's blocked and this will definately ruin a turbo due to excess crankcase pressure.