I've been reverse-engineering this game and I decided to look into exactly how the secret knock works to see what makes it so tricky. I thought folks here might find that interesting as well. I made a video with details and examples which I'll link at the bottom, but to summarize:

• All timing is based on the first knock, NOT the time between knocks.
• Each knock has a 7-frame window, up to 3 frames before or 3 frames after the ideal frame.
• Knock #2 should come 18 frames after knock #1, #3 should come 29 frames after #1, #4 should come 39 frames after, and #5 should come 55 frames after.
• Timing is based on when you press the button, not the animation or sound.

Here's the video with visuals and examples of different knock patterns: