Yes, in French there is a difference in pronunciation between 'u' and 'ou'. http://french.about.com/od/pronunciation/a/ouu.htm
Reference the International Phonetic Alphabet: to an American they are allophones of the same phoneme, but to the French (and quite a few languages) they are very different. "Nous" has your tongue at the back of your mouth, but is still with rounded lips just like "nu" or most usage of "u" in French.
Say "eeeee" but then round your lips as if you were saying "ooooo" while keeping your tongue in the same spot, and you'll pronounce the right sound. In the same way, keep your lips rounded and move your tongue forward and back and you'll hear the transition from u sound of "nu" to that of "nous."