My sympathies! I have a very annoying noise when I first start the car moving in the morning. It goes away eventually - I think when it gets warmed up. I took it to a local Nissan dealer who could not find anything wrong (surprise, surprise!) but did suggest it could be the heat shield (for the exhaust system). This has to be 'loose' so that it can expand when hot, but I think it is fouling somewhere and maybe moving a bit when hot and so stops the noise.
Not sure if this will help but see if 'wiggling' the heat shield (sits above the exhaust pipe) a bit has any effect.
Bill King
November 2009