One of my fuel pipes was split at the point where it goes into the tank. After hard driving and the pressure was equalising, I would get a whine/hum for a few seconds. Can you smell fuel ???
Also, remove the cover (in the boot) just behind the seat. It gives access to the pump etc, see if any fuel is sitting there.
I wouldn't worry about this..had a 2L i Renault a few years back and whenever you switched on the ignition I could hear the pump whining as it built pressure. whining stopped when pressure was built. Bought the car as a 19K miler. Drove it to 100K and the pump was fine. Could be something as simple as the pump body touching something metal and conducting the noise.
Agree with the post above, If it ain't broke, don't try to fix it.