Tricky one Xue,
I don’t think it is a loose body.
I have never seen a loose body that is linear and conforms to the shape pf the pharyngeal neck like this appearance,
My favourite option is that it is a bit of nitrogen gas that has come out of solution due to negative pressure in the joint. This is common.
Another option would be a slurry of some sort of crystal deposition but I think this is less likely.
I don’t think it is a significant finding and if it is gas as I suspect it is normal.
You could scan it again and see if it is still there or disappeared.
Steve