OK, so you've thrown a wedding and invited the whole neighborhood, and you're tired. An hour or so after you go to sleep, all your friends turn out and bang pots and pans under your windowsill, and you're expected to reappear in full wedding attire and feed the rowdies so they'll go away.
Sound like fun? Probably not. But this was the shivaree, which was practiced on the American frontier into the early 20th century.
The American version originated in France, where communities would conduct a charivari for widowers or grooms from out of town. These grooms, outsiders who had effectively snatched a local girl out of the clutches of the local boys, were to pay a toll to the offended locals by offering a midnight meal.
Early French settlers brought the practice to the Mississippi Valley in the 1600s, and other settlers caught on. The midnight parties became an event that grooms worked to deflect; historians cite cases where prosperous ranchers would throw enormous barbecues for the community just to avoid getting "shivaree'd."
Tying the tin cans on the bumper may serve as a poor substitution for an all-night party, but it's interesting to note that the decorating of the car is generally done by the groom's male friends -- men who effectively have lost their chances with the bride who's being whisked away.