Sounded like a exhaust leak

I have a 2012 Altima sedan, it has 40,000 miles
and I bought it brand new. I havent had any
problems until last week. When your inside the car
and it's running while idling it sounds like a exhaust
leak somewhat. I took it to nissan and they tell me
it's this damper thing which is part of the fuel
system.
My car was not having any issues with
performance, it had power as always started up
just fine. There was no check engine light on ever.
The only light that started coming on was the
emergency brake light but would go off after a few
seconds and the anti lock brake light just started
coming on the day I took it in.
Nissan has had my car 3 days and they tell me the
noise is a fuel damper. They are putting a new one
in and will see if that's the issue first but may have
to replace the fuel pump. I'm already into them for
$2,300.00 then they tell me the back right brake
caliper is seized and it cant be removed so it has to
be replaced. I would like to know if this fuel
damper could be the sound I heard. It's right under
the drivers seat and can not be heard when outside
of the car.  I'm worried they might be getting over
on me thinking I dont know anything about my car
so they will do unnecessary repairs.
Anyone have an opinion as to what the problem
may be. They said no exhaust leak so I dont know
what to do at this point.
Thank you
Jessica