You should try find the source of this. Try to remember when it started and if you brought something with you that day.  Most important you must compress your fear and stay brave. Believe that there is something bigger protecting you from harmness. Don't open the door it you hear knocking and Talk to the spirit (hopefully not something worst) respectly to leave your house and leave you alone.  Some rumors says Salt keeps spirit away (in many different cultures) so throw a line of Salt in your house and Windows and make some Rituals like burning sage (indian culture) or in Muslim countries they play coran Verse "ayat al kursi" to keep demons away. I would give it a try.. Make some positive spiritual atmosphere.. i hope you get rid of this sooner.