Loud spring-like clicking above the ceiling, even hours after the AC is off. What could cause it?

[ceilingcheck_assist]

Posting on behalf of the resident: he is hearing a loud spring-like snapping or clicking sound from the direction of the duct above the ceiling. It still happens after the AC has been switched off for several hours.

Here's a 23-second video filmed through the AC vent opening: https://files.catbox.moe/nzts7o.mp4 - not sure whether the sound comes from the duct itself or something else in the ceiling space.

Has anyone in HVAC, construction or renovation come across a sound like this? What could cause it, what should be checked to locate the source, and which type of professional should inspect it?

 

[JohnD]

You would do better to ask on a forum in your own country, where people will be familiar with the installation conventions and the products you use,