You mean this noise?
http://mymicra.com/2015%20Nissan%20M...celerating.mp3
After amplifying it, putting it through a high pass filter, amplifying it again, trimming the unnecessary bits, and listening to it on a loop, I'd have to say that could be anything; if it's the engine, could be water in the gas, or maybe just a loose heat shield...it's barely perceptible. Would have been nice had Robm6981 raised the hood, gently revved the engine, and recorded the sound from there.