WASHINGTON, Oct 18 (Reuters) - Production at U.S. factories unexpectedly fell in September as motor vehicle output slumped amid an ongoing global shortage of semiconductors. Manufacturing output dropped 0.7% last month, the Federal Reserve said on Monday. Data for August was revised down to show production falling 0.4% instead of rising 0.2% as previously reported. Economists polled by Reuters had forecast manufacturing production edging up 0.1%. (Reporting by Lucia Mutikani; Editing by Andrea Ricci) Our Standards: The Thomson Reuters Trust Principles.