- But who would be running a generator on an abandoned airfield?
- I don't know.

4 seconds sound clip from Scooby-Doo, Spooky Space Kook (S1E15).

You can hear this line at 00:06:21.323 in the Blu-ray version of the episode.

Quote context

[...]

- Let's go inside.

- Hold it. Let's settle for a peek inside.

- What do you see?

- It's Scooby. How did he get in there? Hey, Scooby, I'm glad you're out.

- What did you see?

- Nothing.

- You mean the shed is empty?

- I mean the window is too dirty to see through.

- Come on.

- Look at that. The tracks vanish right at the door.

- I see something. A generator. And it's still warm.

- I'll bet you that's what that humming sound was.

- But who would be running a generator on an abandoned airfield?

- I don't know.

- Zoinks! There's that sound again.

- It's the weirdo spaceship! And it looks like it's coming down close to here.

- What'll we do now?

- Come on. Let's split up and see if we can find this mysterious ghost from space.

- Like, wow. We always find the scariest things. But there's no use being scared on an empty stomach. Have a peanut. Take two. They're small.

[...]