The manufacturing of articles or the assembling of unfinished parts by hand or machine or both, except heavy industry, provided that the noise caused by the operation of the machinery, the emission of odors, stenches, dust, smoke, gas or noise are confined within the building or premises where such manufacturing or assembling of articles is carried on, and provided that, where machines are used in such manufacturing or assembling, each such machine shall be operated by a motor of not more than five horsepower. In buildings in which elevators are operated, the limitation of horsepower herein set forth shall not apply with respect to such elevators and conveyors. A blower may be of such horsepower as may be necessary, provided that the noise thereof shall be confined within the building, and provided that the duct shall not extend more than six feet above the roof level of the building.