2001 Volvo V40 diesel engine, high-pitched jet engine sound when accelerating?

From the symptoms you mentioned earlier on in the question I thought It was the turbo. on old cars, or even new cars when the turbo gets old it will make different noises. (The sound of a jet engine is normally ociated with a turbo spinning up to speed.) As long as the noise isnt getting progressively worse you shouldnt worry. But if you know someone who could inspect the turbo seals for leaking,(even if you have to pay him) and the turbine for shaft play it would be money well spent. (in the event that it needs replacing)