Don't Come A-Knockin' - Television Tropes & Idioms

...when this van's a rockin'!

A form of G-Rated Sex where a vehicle or other small enclosed space is shown moving vigorously to show that its occupants are engaged in vigorous activities of their own.

A common subversion is to then show that the vigorous activity in question was not the one you were thinking of.

Sister trope to Auto Erotica.

Examples:

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Commercials

The famous Peugeot 207 commercial with ladybugs.

Film

In the early Jim Carrey movie Once Bitten, a coffin is shown rocking to show that the hero's girlfriend is finally doing the one thing that would end their current problem...
- Subverted previously when, out of the plethora of other cars at the Make-Out Point, Mark's ice cream van was the only one that wasn't rocking!
Big Fish has a rocking trailer, but it turns out there's just a Hell Hound inside.
A van is rocked this way in the MST3K feature Hobgoblins, much to the hosts' disgust.
Tom Servo: They made love in their Chevy van, and that's not all right with me.
- Don't forget that the car made cartoon bouncing noises with each bob.
Occurs in the opening of Idiocracy.
Used in the film version of Harry Potter and the Goblet of Fire in a carriage. Snape comes a knockin'.
Titanic: In a car that's on a ship.
Subverted in Whip It, the dad's car is seen doing this, accompanied by loud shouts from the dad. Turns out he's watching a particularly exciting football game.

Live-Action TV

On Bones, Booth and Brennan went undercover as circus folk, and to pretend they were a couple, they rocked their trailer back and forth to make it look from the outside like they were doin' it.
Circus Manager: Gotta give 'em points for stamina.
Pushing Daisies: In the second episode, there's a flashback of the Victim of the Week, a car safety tester, meeting his girlfriend, a promotional model for the same car company. This is followed by the display car in the showroom rocking back and forth, with the windows steamed up. Bonus points for the car being on a turntable.
- Also referenced in the second episode of season 2. Chuck notices the bumper sticker "Mimes do it with imagination" on a van just before they open it to find a dead mime. Emerson comments that the bumper sticker should have said "If this van's a-rockin', I'm being murdered."
Subverted in Mama's Family. In the episode "There Is Nothing Like the Dames", Thelma is trying to impress a small group of snooty upper-class women with a backyard dinner so they will let her join them. Unfortunately, Vinton and Naomi's trailer sits in the driveway, an obvious eyesore. On top of that, during one scene, the trailer starts rocking, horrifying the women... until Thelma finds out that Vinton was only trying to fix a broken table leg.
Seinfeld. In one episode, Jerry gets a van from an old friend. Elaine suggests that Jerry should outfit it with one of the troponymous bumper stickers. Also, near the end of the episode, Jerry and George catch George's parents rockin' the van.

Video Games

A less explicit version is in Grabbed By The Ghoulies, where the hero is sometimes chased by walking coffins that conceal plump, amorous vampire girls. If one catches him, he's yanked inside, and the coffin rocks for a few seconds (With ♥s drifting out and everything) before letting him go.
Pretty much all sex—I mean "Woohoo" done in The Sims.

Web Comics

Subverted (though not in the usual way) in Questionable Content.
Pintsize: If the router starts a-rockin', don't come a-knockin'!
Pintsize: ...Actually, if the router really does start a-rockin', do you think you could weight it down with something heavy? I don't want a cable to come loose and disconnect me.
Subverted in It's Walky! when Walky and Joyce try to convince Joyce's grandchild-obsessed mother that they are having sex: the two of them simply bounce on the bed loudly like the overgrown kids they are.
The gag itself is subverted when Joyce has an epiphany about why the two of them hadn't slept together yet.
Tycho Brahe: Mr. Coulton? Um I know the trailer is rocking, and I really shouldn't knock in this instance, but... they need you for sound check. They want to check your sound.

Western Animation

Subverted in The Simpsons:
Homer: Son, I've had some of the best moments of my life in the back of a car.
(cuts to a picture of a rocking car)
Young Homer: Oh yeah! etc
(Interior shot of car reveals he's eating junk food)
- Played Up to Eleven in the episode where Homer visits Marge in prison: when Marge jumps on Homer, the trailer gets knocked over.
There was a Getting Crap Past the Radar version in Two Stupid Dogs. The two dogs went to a Drive-In Theater, and every car around them was rocking.
Referenced in Animaniacs, where the Warners drop in on Dr. Scratchensniff and his date. Eventually, they start bouncing around, causing the car to jump as well. Outside, you see a mother forcibly turn her young son's gaze and push him away. *mwah!* G'night Everybody!
On The Venture Brothers, The Monarch and Dr. Girlfriend first have sex in a car. They even manage to steam up the windows enough for the Monarch to write on them.
Somehow, this was able to slip past the radar in Total Drama Island, in this case Owen said 'boat.'