ByJohn S. McClenahen Like the manufacturing sector of the U.S. economy, the service sector continued to grow in September but, as expected, more slowly than in August. The non-manufacturing business activity index compiled by the Institute for Supply Management (ISM), Tempe. Ariz., was 63.3% last month, down nearly two points from record highs of 65.1% in July and August. September was the sixth consecutive month that the ISM index has shown the service sector growing. Growth was broad-based in September: 14 industry groups, including wholesale trade, transportation and insurance, grew; none contracted and two remained unchanged from August.