Grinding, rumbling noise during acceleration

My 1997 Rover 200 SDI developed very annoying feature. When I cruise around 80km/h at 5 gear and then press acceleration pedal I hear very loud rumbling, kind of roaring and grinding noise. ONLY when accelerating. I can hear it at lower speeds but at 80km/h you can't miss it. I can even feel it on the steering wheel at faster speed. Accelerating round 120km/h makes it's so loud I feel like it's going to break apart. I suspect it has something to do with gearbox (there's oil inside, full) - but what, bearings? And why it's only present when I accelerate? When I'm cruising at constant speed there's no noise or soever.

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