Might be your turbo on its way out. A friend's Volvo C70 High pressure turbo Convertible did the same thing right before the turbo failed. Not a very common thing to happen especially on a diesel. You might invest in a stethescope and place it on different parts of the engine while at various RPM's to see exactly where the noise is coming from. Belts and other parts can scream as well. Good luck. Hopefully its a belt or tensioner rather than a turbo.

Oh also on the stethescope thing use a bit of caution. Ears are very sensitive to loud noises.