[2020] Bolt Search Event Puzzle #7 - Knock on the Walls

As you enter yet another crypt looking for bolts, you suddenly notice a bolted steel door in a stone wall. As you get closer, you hear a weird, high pitched sound that resembles a knock coming from the door.

There it goes again.

What could it mean?

Results

As always, the first person to solve this puzzle gets a hint. Puzzles are getting harder now, so good luck!

WINNER: /u/TheBes7

Hints

Hint #!: NNNddUU