Weird sound in a rear internal hub

Hey guys! I got a new ride two weeks ago - Cube Hyde Race 2017 (Gates belt drive, internal Alfine 8 hub). Been waiting for it since July. Loving it so far but can't help but notice a weird noise when I ride it. Since the model is so niche it's hard to find anything on forums regarding it. So whenever I'm riding on a uneven surface (even if it's just rough tarmac) there's something in the frame (can't tell precisely as it resonates) that does a chattering (ding-ding) sound somewhere around rear hub or bottom bracket. It sounds like a loose bolt or something (metal on metal). I never had an internal gear hub so it's hard for me to tell if it there anything inside that has to make this sound. I checked everything on the outside of the bike and it all seems to be in place and nicely screwed in. Maybe you guys had something similar? Cheers!

Upd. I've recorded a quick video https://youtu.be/BCdA9fOIq2A

I doubt very much it's inside the rear hub. Something "bouncing" around inside would surely trash an IGH. I reckon it could be something externally rattling when the road is uneven.
try bouncing your bike in a controlled manenr when you're standing beside it.

Internal cable routing?

Well, that's probably the case. Something inside the frame. An internal cabling wouldn't (in my opinion) sound like this. I've recorded a short clip where you can clearly here it. https://youtu.be/BCdA9fOIq2A

is the Gates cog on the alfine definitely secured? Are those allen head bolts securing the dropout def tight?
If you lift the frame off the ground and shake it around can you hear it?