Place it just before the reveal in your edit, then follow it with the Shocking Sound Effect once the hidden thing appears. If the box turns out empty, use Awkward Crickets instead for a comic letdown, and keep the next reaction sound close behind it.
Über diesen Sound
This is a short Polish voice line, lasting just over five seconds, in which a speaker says something is inside here. The delivery is quiet and watchful, turning a plain observation into a moment of suspense. It feels teasing and slightly unsettling, as if a secret is about to be found, making it easy to picture a hidden object, person or surprise waiting just out of view.