Mine does it too, I think that at one point in time all honda's do it. The sound is the fuel pump but why it sometimes doesn't cut on is beyond me. I eliminated the problem by always listening for those sounds, and if I don't hear them I just turn the key back to the "off" position then on again.
One day I'll look into it and find out what it is but a quick fix is to just make a habit of always listening for it..

Although I can understand your concern as I sometimes get worried the pump will fail alltogether.