Recently purchased Navigator as liked the experience during the road test and my wife wanted to try out American luxury vehicle. So far liking everything except for the annoying whistling sound that becomes noticeable as soon as car climbs over 55 km/hr. I arranged for dealership to diagnose the problem. However, after testing it today, they can't hear the sound (which has been detected by all members of my family either driving or riding in the front seat). I told them to keep at it until they can identify the issue but so far it seems to go nowhere. Does anyone have any suggestions on how to make sure this issue is recognized by dealership and dealt with? Thank you in advance.