In August, the prices American producers paid for goods rose exactly as economists expected — yet the calm surface of that number conceals a more restless current beneath. Energy costs, led by gasoline, surged to their highest monthly gain in three months, a reminder that inflation does not always announce itself loudly before it arrives at the consumer's door. The Federal Reserve, still navigating the narrow passage between cooling prices and preserving growth, now faces a supply chain that is not cooling as gracefully as hoped.