1 {Monty leaps from horse on to truck}
2 {scrambles for a hold}
3 {clings on precariously to outside of truck}
4 Ginny: You could have just jumped straight in the back here, you know. I'm not going to stop you.
4 Monty: Now you tell me!

Sometimes there are just no suitable onomatopoeic sound effects that you can use.

Of course, there's a TV Tropes page on this. You have been warned; I made that link explicitly tell you where it was going so you can avoid the hours of exploring you will no doubt end up doing if you click it.

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2019-03-31 Rerun commentary: The proper sounds here are of course some sort of John Williams musical score.

Hands up if you can now hear it in your head.