Dual flush toilet cistern makes whining noise?

Just a thought it could be slight vibration causing the float to vibrate commonly known as 'knocking' some call it 'Bounce' in my ancient plumbing school.
Try this dodge I've used on main loft tanks floats stick a polystyrene tile or similar under the ball as large a piece possible without fouling the cistern walls and see if that stops it. Can't think of anything else except the valve may be low pressure [Tank fed] and it should have high pressure valve [Mains fed]

Nobby
April 2008