its normal. this is what the guys at honda say.....
"Hum or Moan Near
Fuel Tank?
On any Honda except a Passport, if the outside air
temperature is high, and there’s not much fuel in the
tank, you may hear a hum or moan near the tank or in
the engine compartment, especially when the vehicle is
parked with the engine off. More than likely, this noise is
the EVAP two-way valve doing it’s job. Located in front
of the fuel tank, the valve vents excess fuel tank pressure
to the EVAP canister.
To verify the cause, take off the fuel fill cap when you
hear the noise; if it goes away when you release tank
pressure, the noise is coming from the two-way valve.
Don’t replace the valve because of the noise; it’s a
normal characteristic."