I can hear the fuel slosh in my 97 Civic. I think it's like Hot_Georgia said; the car is so quiet that it's just easier to hear. It's also possible the hybrid has less sound deadening material in that region either due to space constraints with the battery pack or in an effort to reduce weight. The old Integra Type-R had an unbaffled tank as a means of weight reduction. It's possible they have done the same with the hybrid to save a couple pounds.