I have a 2010 g37x , check the steering assembly for any signs of leaking around the boots. They are well known to go. These cars don't have the typical rack n pinion assembly. Its a all in one unit. I had mine replaced. Its something worth checking, as it is a known issue, and expensive.

 

Chiming in here since I have a 2012 that had this problem.
Check your rear splashguard/undercarriage plastic. One bolt towards the front is broken holds a huge portion of the plastic. If it breaks, air can catch this piece and cause this.
In mine it was happening intermittently, anywhere between 55-85mph, usually after I hit a bump, and continued until I coasted down to 40mph-ish.
I only guessed this was the answer cause I broke the bolt myself. If it rusts off or someone accidentally sheared it, this issue would occur.
Once I slapped a new bolt on, iiisue hasn't reoccured.

 

My ‘15 Q40 AWD has intermittent vibration in the front seats and rear view mirror at 75-85 pretty much everyday before its fully warmed up. After 5 miles or so on the interstate it diminishes and eventually goes away. This tends to be noticeably worse when leaving work (<1 mile to the interstate) in the evening the first few miles.

I have road force balanced the tires which helped a slight amount. From what I understand this vibration is most likely caused by one or more of the drive line bearing sets in the transfer case and/or transmission being ever so slightly imperfect. Once the bearings warn up to operating temperature they expand reducing the tolerances and hence the vibration.

Apparently this condition is expected to get worse with time/mileage as the bearings wear. So we have that going for us. I can attest to that as I now have 63K on the ticker and it is a slightly worse than it was at 20K. This is the best theory I have heard in regards to this annoyance. It’s not to the point where I want to start replacing bearings. So I’ll live with it for now.