Acceleration noise with new 2015 rogue sv

[RCT13A]

Hell0,

I bought a 2015 rogue sv three months ago and about a month ago started to hear a rattle or fluttering noise with acceleration. The noise is only present with going up a hill or when beginning to accelerate. Noise ignores away after a certain speed. It is not there when coasting or deceleration. It is has been at the Nissan dealer for 2 days and they have called corporate and can't find anything wrong. The mechanic said he moved the heat shield and found some screws loose and stripped. He fixed that part. But it still make noise. What should I do? Anyone else have this problem? Thanks Bobby