G37 cat rattle?

I have a rattle coming from the passenger side. It doesn’t rattle at idle, only when the car is in motion. It’s not reliant on rpms or if it’s in gear just whenever the car reaches 10 mph or above I start to hear what sounds like metal pellets or maybe pebbles inside of a metal container bouncing around.. the faster I go the louder it gets. When I’m on the highway and come next to a car I can hear the noise bouncing off from the car and it’s loud. It’s like the sound of you were driving over gravel at a fast speed and you just hear a bunch of pebbles hitting and bouncing around metal parts... I’ve taken it to an exhaust shop and they banged on everything. Exhaust, y pipe, wheels, heat shields, cats. I’m almost certain it’s the cats as I really cannot think of what else it might be. I’ve heard the material inside cats can become loose and rattle internally. But I’ve watched videos on it and the rattles from a cat where the honey comb material comes apart sounds like it’s one big chunk making a knocking rattle rather than the rattle I’m describing that sounds like someone is shaking the **** out of a tin can with pellets or pebbles inside of it... goes away when I come to a stop and always makes the noise when I’m anywhere above 10mph. Even if I’m in neutral coming to a stop or accelerating/ slowing in gear... the exhaust shop could not find the problem so I’m going crazy trying to figure it out.

I removed the y pipe heat shield thinking it was that but I went on a drive without them and the rattle was still there. One of them mentioned pebbles could be stuck between the brake dust protecter or in the rotor itself.
I have a hunch it’s the cats and plan on getting a pair of hfc’s but I’m skeptical as the rattle noise is different than the videos I watched of cats with that problem.
Any ideas of what the problem might be?

I will take a video soon to attach here.

 

Did you ever find the source?