Nissan Repair: noise after driving 15-20 minutes, infiniti qx4, waranty

Question
I actually have an Infiniti QX4 2001 but could not find that as an option.  Anyways, the car makes a kind of beeping noise, hard to hear with background noise, but obvious when I pull into my garage.  It is after I drive 15-20 minutes, more in warmer temps and longer rides.  the noise is an intermittent beeping noise that lasts 1-2 mninutes but will stop if I open the gast tank and let out air.  Infiniti will not do anything even though I am under waranty because they said they cannot duplicate the noise...well it is so loud in the garage that they would not be able to hear it.  I let the guy drive my car home to his house so he could hear it in his garage, but of course he said he didn't hear it.  It has done this since I bought it new!!!  I wish I would have insisted on a new car right away!  Any ideas for me to help the mechanics figure it out and fix it?  before my warranty runs out!  Thanks!

Answer
Sue,

You got me, I would say take it back to the shop and tell them to fix the thing (demand it) or replace the car (demand it) since you are still under warranty.

Don't let it go!! exchange the car.  While you still have the warranty.
The only thing I could think of is a back up alarm, if there is one, it could be ging haywire.
Sorry couldn't help much
Darrel