Sorry - should have gone into more depth.
The noise is a sort of deep rumble coming from the engine, almost like over-revving but with more of the lower tone you get at lower revs which I find quite odd. Car's got full service history with a reputable local garage, both I and the garage originally thought it was a turbo issue as it seemed to tick all the boxes but the turbo was removed, inspected and cleaned with no issues found other than a bit of carbon build up. The noise was definitely improved for a little while after this had been done but came back pretty quickly.
