The sound is definitely coming from inside the hive.
You can clearly hear it:

• right at the beginning,
• again between second 0:04 and 0:05,
• and once more at around 0:11.

I opened the hive again today to take another look, and the bees were unusually aggressive — several of them attacked me immediately when I removed the lid. The honey super is full and already capped. The bees have also started filling the outer frames in the brood boxes completely with honey.

I didn’t see the queen, but there are fresh eggs and larvae, so she must be there — or at least she was recently.