Turbo fault ?

I have a motorcaravan based on a Fiat Ducato 1.9 TD. Less than 30,000 miles over ten years.
The engine has recently started producing a strange howling noise, but
only when the vehicle is in motion. The pitch of the howl rises and falls with engine revs and while working up the gears can produce a fair imitation of a police siren! Once cruising in top gear the noise is barely audible.
Given the symptoms, I am assuming that the noise is attributable to a turbo fault of some kind - possibly an air leak somewhere - but because the noise is only there while the vehicle is in motion it is rather difficult to pinpoint the source.
Has anyone experienced something similar with a turbo diesel Ducato ?
George Waring
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