May be none. Up to half of people can have no symptoms. It can be found by accident when a chest film or ct scan is performed and a mass is found.Since it is the middle of the chest and if it grows large it can cause compression symptoms such as trouble swallowing or chest pain. It is looked for with certain diseases: myathenia gravis and can be associated with connective tissue and autoimmune diseases.
Usually none. They are often asymtpomatic. When they get large, you can get pain and some shortness of breath if they involve the phrenic nerves.
This may help. Space limited. These links may help you: http://goo.Gl/kx1gl and http://goo.Gl/oetwz.