Your Linden has a gall which is very common. It is unattractive but not a serious problem for the tree. Here is a link with more info about tree galls. http://www.extension.umn.edu/garden/insects/find/insect-and-mite-galls/
This same tree gets a ton of Japanese Beatles each year. Can you suggest a way to address this too?
Here is a link that explains the Japanese Beetle life cycle which is important in dealing with them, they are a nasty problem. http://www.extension.umn.edu/garden/insects/find/japanese-beetles/