A lot of people can recognise certain engines from a distance. Not so difficult when we drive trucks with such noisy lumps. But some 300tdis are recognised by their incessant squealing.

This can be a couple of things but in most cases it is a worn tensioner pulley creating the background harmonies.

From the photo below you will see that there is a thin rubber gasket between the tensioner body and the moving arm, or at least you SHOULD be able to see the rubber gasket.

It is clearly visible in the new unit in the photo (bottom) but worn away completely in the old unit (top) This allows the metal surfaces to rub harmoniously together. It is easy to replace the tensioner but they are not cheap.