There are many things that can cause a vibratrion in an engine. Yes some is normal and havingit idle low would cause it to be worse. A unbalanced prop can cause it a misreamed prop can cause it a loose engine could cause it. These are all things I would look at. Also look at what seems to be vibrating. Does it look like just the engine is shaking or is the whole plane. does it get worse with higher RPM or go away.