I was having a similar issue where I opened a thread here:
Rev light stall on AT
For the quick and dirty the Nissan Dealership told me it was pretty much downshifting but I was hitting the throttle to hard (yes, according to them even in automatic you can hit the throttle to hard) and so it hits the rev limit and you get that half a second stall with the sound. He told me in that instance I'm about to accelerate hard and I'm around 60 to pay attention to the rev meter and if looks like its about to limit just ease on the throttle and then go back hard on it if I want, or just be in manual mode, or finally he advised maybe I should just go get a GTR.