2004 Toyota Highlander Owners Manual - Page 416

'04 HIGHLANDER_U (L/O 0308) Leak detection pump (2AZ −FE engine) This pump performs fuel evaporation leakage check. This check is done approximately five hours after the engine is turned off. So you may hear sound coming from underneath the luggage compartment for several minutes. It does not indicate a malfunction. viii 2004 HIGHLANDER from Aug. '03 Prod. (OM48478U)