2011 Lancer: Weird noise that sounds like loose metal when on a constant speed
 

I have a 2011 Lancer ES CVT 2.0L. I need help in a situation that I am having a hard time figuring out a noise that is happening all the time when driving at a constant speed

When I start to drive and accelerate from a stop, I cannot hear the noise at all, but when I'm at the speed and the RPMs drop to around 1500 RPM, the noise starts to happen, It sounds like some metal is loose and it is beginning to shake somewhere. When I accelerate again with over 2000 RPMs, it goes away. Also, when I rev it around 1500 RPM in Neutral, the noise does not occur at all! The noise is mostly coming from the middle/passenger area somewhere behind the dash, but I cannot figure out exactly where the source of the noise is coming from. To make this easier, here is a video of exactly what is going on with captions on what I am doing. I hope you all are able to hear it since I was able to compress some of the background noise and make the noise easy to hear.

Here is what I have done. I opened the hood and checked for anything loose, nothing. Everything looked good. I thought maybe it was a heat shield, based on some other posts I have seen online, I checked thoroughly on the exhaust shields and there was nothing loose!

This is not affecting the way it drives. This has been happening for over a year now and I still cannot find the source of the problem. I hope some of you can figure out where it is coming from.

Thank you in advance

I had something similar which ended up being a loose piece of the plastic skid plate rattling around. Not sure if you have looked under the car yet? Hope you find it.