Sounds strange to me too.
Maybe some of the spray got to bearings for a temp fix, but never heard of a belt making a rattling noise. Especially a noise like in the video.
Edit: looked at the video again. the belts didn't look loose so I can't see the belts being an issue. The rattle sounds more metallic in nature.
Pretty sure a bearing IMHO.
But I guess if the "rattle" went away, I guess it's good... for now.