2005 Toyota Matrix Owners Manual - Page 643
2005 Toyota Matrix Manual
Page 643 highlights
'05 MATRIX_U (L/O 0409) Leak detection pump (two −wheel drive models) 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. ix 2005 MATRIX from Sep. '04 Prod. (OM12892U)