Whoosh-bang sounds a little disturbing if it is very evident. I worked on
an Elantra that had a baffle come loose inside the fuel tank.
But I'm in no position do doubt someone who actually heard the noise and
checked into it, either.
If you're concerned about the noise, the dealer should be able to explain
to you the cause or, if they don't know the cause, should be able to
demonstrate the noise on another identical vehicle to appease you that it
is a normal condition.