There are quite a number of causes of squealing coming from an engine. Some more information about the circumstances of when it occurs will help.
It could be drive belts, it could be tensioner wheels. It could even be a leaky exhaust manifold gasket making a whistle or a scream from the turbo.
We would need more information in order to help, such as at what engine speed it starts, does it change with engine revs and so on.