Yes, I know the noise you are talking about. 50% or so of the time. It seems like its more prevalent when the car is cold. Kind of like a flapping noise. I can't track it down either. It almost sounds like a trim piece or a wheel well rubber flap. Goes away after the car warms up
Nothing loose though. Very frustrating.
I do suspect something though. There are a couple of metal braided hoses on the exhaust, right under the engine. Since the car seems to make the noise more often when cold, I think that those connectors are thermally growing (contracting) when it heats up.
Not sure though