Looking for ideas,

I bought a 2008 Xterra a couple months ago with about 14,000 miles on it. After a few weeks I started hearing a high pitched whining noise (kind of like a snowmobile or chainsaw) upon accelerating mainly around 10 and 20 miles per hour. It does it quite often, but not all the time. It only lasts for a second or two, but of course when I bring it to the dealer (an hour away) it doesn't do it. Any thoughts on this would really help!

Thanks