Toyota Tacoma 2006 Operating Manual page 416

06 01.06
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 deck for several minutes. It does not indicate a
malfunction.
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2006 TACOMA from Jan.'06 Prod. (OM35865U)