[crossposted from the other site]
What year car have you got? This sounds to me suspiciously like the uneven tyre wear "cupping"/"featherering" problem (not actually either of those in reality as far as I understand what cupping and feathering really is, but that's the closest proper name, and what it's referred to as in most websites.) Anyway,
A description of one guys experience:
"How do you know if you have this problem? In my case, I noticed very loud road noise at low speeds, somewhere between 20 and 30 mph. This noise started some time ago but it gradual got worse to the point that I decided it was time to investigate. By this time, I had 5000 miles on the vehicle. The road noise sounds like a grinding roar and is predominant at low speeds. By the time you reach 50 mph, it is inaudible. There was no pull to one side or the other. There was no thumping, just a cyclic roar. At first I thought it was a wheel bearing going out. As I inspected the tire, I ran my hand over the tread and felt the abnormal tire wear as described above."
See e.g. here:
http://www.my350z.com/forum/showthread. ... adid=15758
http://www.nissantireproblems.com/resources.html
http://www.lasercannon.com/Newz/feathering.htm
Some other sites/people have also mentioned that it appears worse when braking, which also fits with your description. Anyway, there are technical service bulletins about this problem, so you should be able to get it sorted under warranty... with a bit of luck... The good news is it, as you say, does not affect the handling or safety of the car, however, the noise and tyre wear is just damn annoying. (Or so people say...)
Hope this helps (and hope I'm wrong and that your problem is rather more trivial!)