I would think its one of the belt tensioners or the polly V-belt itself.
You should open the hood and film this sound again. Moving around the engine area. Did you have the AC on? Or does the noise change with the AC on/off?
I have a wierd fluttering noise only after driving for atleast and hour or so. And it only makes a fluttering noise as teh revs fall between 1,800 roms to 1,200 rpms. But it never makes teh noise for my dealer.
I suspect its the belt tensioner (bearings inside of the pulley) that are going bad.