WASHINGTON—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 percent last month, the Federal Reserve said on Monday. Data for August was revised down to show production falling 0.4 percent instead of rising 0.2 percent as previously reported. Economists polled by Reuters had forecast manufacturing production edging up 0.1 percent.