Had to write and post this, because its a thing that no-one seemed to have heard about.
Laguna MK1 2.0L 16V 2000 Estate 115000 miles
I had started to feel a vibration between 45 - 55 MPH. I thought that it was a wheel balance problem but noted that:-
I swapped the wheels front to back - Still there.
Its like a side to side shaking. I can almost see the nose twitch. Whereas wheel balancing is more up/down.
Can't feel it through the steering wheel.
It only happens while power is on. It increases with added power. I THINK that the car tries to slow down while the shake is on. Coast or brake and it goes away.
I can't feel excessive play though the steering wheel or in the wheels when I jack it up. Tyre pressures are good. No wear pattern. No slack in the driveshafts or droplinks.
The left hand driveshaft locates into an output bearing. The case hardened surfaces had worn and broken up in such a way that the bearing in the shaft locked into it under power (wear patch 6mm wide 1.5mm deep). When combined with the angle of the shaft, the rotation worked the bearing like a cam - pushing and pulling it, and the engine that it was attached to!. That speed corresponds to the resonant frequency of the engine and box on the rubber mounts.
The debris was noticable in the CV boot. If it had got worse the debris would have totalled the gearbox. New bearing £86,
£25, new drive shaft nut £1.50, not having to worry about the driveshaft tearing abart at 50MPH - priceless