In all residential zoning districts and in a Neighborhood Commercial District, no truck-tractors, semi-trailers or combinations thereof shall be parked or stored outdoors, on or off of a public street or highway, except for the purpose of loading or unloading. This loading or unloading must be easily verifiable by code enforcement personnel, and during such times of loading or unloading, such truck-tractors, semi-trailers or combinations thereof shall not be permitted to have their engines, generators or refrigeration units of any sort in operation for a period exceeding a total of 30 minutes in any twenty-four-hour period.