Knocking sound when tilted - around top of keyboard or screen area

There is a distinct knocking sound when tilting the Macbook. It feels like something is sliding and knocking the metal casing, but it's extremely had to identify exactly where. It sounds and feels like something heavy, possibly a battery, but the batteries are towards the front (and stuck down!) The knocking sound is coming from the bottom of screen area, more towards the right but across the length of the hinge.

This has baffled me and there appear to be more people with this issue - is it the fan moving on it's axial?

Is there something loose in the screen itself? - it does sound like something heavy, flat and moving side to side (it's absolutely not a loose screw or something 'rattling' around).

Is there a solid metal bar/fixing in the hinge area it could be?