Rattle inside frame: cause for concern?

Recently, something started rattling inside of my less than a year old hardtail mountain bike frame. I was unable to figure out what, and could not fix it. Is this cause for concern? The frame is made of aluminium.

The rattling is audible when riding over rough stuff or cobble stones, when tapping the side of the frame, or when shaking the frame hard with the fork off. There is nothing sliding around in the tube, only rattling.

This frame does not support internal cable routing (other than the dropper), so it is not that. It sounds like it is coming from around the middle of the down tube, or possible top tube. I took out the seatpost to try to take a look, but the seatpost does not communicate with the top tube (other than through a tiny drilled hole). I took off the fork, where I can see an opening into the tube, but there is nothing visible there.

I don’t think I can do anything about this rattle, thus my main question: do I need to be worried about it, could it indicate a serious problem? Or should I just try to forget about it and live with it.