If you have professionally installed siding and roofing this should not be a problem. If you are having this problem the most likely place for water to seep into the wall cavity would be from the roof. Water can find it's way through almost anything and sometimes these kinds of leaks can be very difficult to locate. Have a professional roofer check the roof over to see if there is anything obvious. They will check the valleys, areas around the chimney, around plumbing vent pipes and roof vents etc. It could be a simple fix or it could mean replacing the roofing materials.