Toyota Tundra 2005 Owner's Manual page 501

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Leak detection pump
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 under-
neath the rear deck for several minutes. It does not indi-
cate a malfunction.
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'05 TUNDRA_U (L/O 0409)
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2005 TUNDRA from Sep. '04 Prod. (OM34432U)