Does it do it all the time while driving on I'm assuming bumpy roads? How the heck can't they hear it yet you do, are they on overly flat roads? It may be worth reaching out to infiniti customer service. They will most likely take your info and story about how the dealership isn't willing to investigate it at all, even with the video/audio and have a regional rep/manager get in touch with you. The number is 1 (800) 662-6200.