- You're the only one who knows.
- Your secret is safe with me.

3 seconds sound clip from Superman (1948) - Man Of Steel (Chapter 4).

You can hear this line at 00:06:17.511 in the Blu-ray version of the episode.

Quote context

[...]

- If you are really Superman, you have x-ray vision. Tell me what's inside that closet.

- That's easy.

- My x-ray vision has been destroyed. Must be the effect of that meteorite.

- No. That door and the closet are lined with lead for use in certain experiments. Even x-rays won't pierce lead.

- Try the safe. It's only steel. Tell me what's inside.

- It's filled mostly with papers, scientific notes. There's also $500 in currency. Yes, and a couple of postage stamps.

- And now, if you want further proof...

- No... I'm satisfied.

- You're the only one who knows.

- Your secret is safe with me.

- Now that I'm convinced, what do you want me to do?

- For years I've suspected that I came to Earth from the planet Krypton. And now, this meteorite seems to prove that. It takes away all my powers that makes me superior to Earth men.

- There's no doubt about it.

- Washington has selected me to guard the relativity-reducer ray.

- Nothing must interfere with my ability to discharge my duties.

[...]