Could be the dual mass flywheel if the rumbling goes after pressing the clutch pedal down. They're wobbly things, even when new, but get progressively worse with mileage and/or driving style. DMFs hate being laboured (which makes it an odd choice of flywheel for a diesel!) and over-revving, so there's a bit of a operating window to keep them in if they're to last.
Perhaps when turning right, the DMF is tipping over away from the engine slightly, if that makes sense. I think it might have been an early humble mechanic video, but he showed where a severely worn DMF was actually contacting the gearbox bell housing, which cause a nasty thumping/rumble.
Anyway, things might not be that bad on yours but worth getting it inspected to outrule/confirm it.