I have the same problem/symptom with my GX. Essentially heard it since day 1 that I had the car .. the dealer "could not" reproduce it though when I brought it back at the 1000 mi service. It appears to be some kind of rhythmic cycling sound .. as opposed to a constant drone when accelerating. I only hear it when accelerating slowly or applying gas between 70-80mph. Above 80mph it seems to go away. And I don't hear it if I am blasting through to 80mph quickly (like from 40-80 .. quick accelerating on the freeway). I can't say that I hear it all the time .. but most of the time .. and I can't say that it is affecting the performance of the car .. but it is annoying if nothing else. I plan on bringing it back up with the dealer at some point. I have had the car for a month and have 1200mi on it
