It couldn't be simpler.
2 seconds sound clip from the Back to the Future Part III movie soundboard.
You can hear this line at 00:48:37.840 in the DVD version of the movie.
Quote context
[...]
- What does this mean? Point of no return.
- Here you go, Marty. Connect that to the positive terminal.
- All right, Marty, you all set?
- Pull up to the DeLorean! Pushing the DeLorean up to 88 miles per hour!
- It's Clara. Quick, cover the DeLorean!
- Hello.
- Why, hello. This is quite a surprise.
- Well, I hope I'm not disturbing anything.
- Oh, no, no, we were just doing a little model railroading.
[...]