Atlas Honda Accountants take too much of salary etc, thats why it makes economical sense to them to remove any such items like
Sound deadening
Trunk light
Side Air bags
Tank Buffer
They know we Pakistanis have this "aura" of eminence while driving a Honda City - so it doesnt matter if it has sound deadening - at least they look cool in their minds while the car looks complete from outside (Honduh Standards)
Now to tackle the problem
The fuel tank sits under you and the passenger - Cant fix that - its pretty sturdy PA-GF plastic - dont worry it wont rupture (most german cars come with plastic tanks too - no rusting issues) - it also quietens sloshing sound (can you imagine sitting on a metal tank with fuel sloshing in it - ouch)
you need to get hold of some proper sound deadening mats (Dynamat) and stick it to the floor and door insides, and a piece of foam (or liquid spray styrofoam for the tank top)
It will dampen any echo you complain of - Do also make sure that your stick on door plastic curtain sheets are not missing. (Atlas Honda may also put this in the above list)