Why is my Vaillant ecoMAX828 so noisy?

The boiler works fine when firing up for central heating, and also for medium demand hot water. However, when I turn the water up full a loud vibration starts, but ceases again when the flow is reduced. It has a new fan and diverter valve fitted, and has been flushed through, but the fault persists. The gas engineer is stumped, as are the technical advisers at Vaillant, it seems. The bill is mounting up and a solution seems no nearer. Any advice would be gratefully received!