The pilot has just blipped his motor. That means he's dropped the flare.
5 seconds sound clip from the The War of the Worlds (1953) movie soundboard.
You can hear this line at 00:23:38 in the DVD version of the movie.
Quote context
[...]
- There's been a lot of mysterious activity around the machine.
- Lights and dust, as if they're digging themselves out.
- If this keeps up, it'll be a guide for the plane when it comes over.
- The pilot has just blipped his motor. That means he's dropped the flare.
- He's flying high, so it'll take a few seconds to come down.
- They're after the plane with their ray!
[...]