Banging for Help

The hero is Bound and Gagged. With no avenue of escape, and severely restricted movement, what is there to do?

Well, assuming there's no Conveniently-Placed Sharp Thing nearby to cut the bonds, there's only one thing to do: Get attention in the hope of attaining rescue.

The simplest and most common (and often, the only) way to do this is to make noise. Banging on a wall or floor with bound feet, or making noise through a gag, are the most common. If the captive has a bit more freedom of movement, then knocking things over could work, or making a signal of some kind. If the captive is lucky, which happens the majority of the time, it'll be the heroes who hear the noise and come to the rescue. Sometimes, it's instead the villains, who then do a better job of restricting the captive. Other times, there's nobody around to hear the captive's attempts at rescue.

Note that merely struggling and making noise out of frustration is not this trope. Only if the captive is making noise with the goal of obtaining rescue, would it be Banging for Help.

If the captive is more savvy, he or she will bang in Morse code, which ensures that somebody will come by and realize what the message is and come to the rescue. Because, you know, Everyone Knows Morse.

Examples of Banging for Help include:

Anime and Manga

Happens several times on Case Closed to alert Conan, who has Hyper Awareness, that somebody's in serious trouble somewhere.
Fullmetal Alchemist: when they got trapped together in a place where Pride was powerless, he pretended to play with Al's helmet, but his idle banging actually contained messages to his allies.

Comic Books

A variation occurs in the Astro City "Dark Ages" story arc. Seeking shelter at an arms cache during a citywide riot, Royal Williams finds his brother Charles dying from a gunshot wound. Desperate to attract the police despite the chaos, Royal fires off all of the weapons to try and warrant attention.
An issue of Catwoman has the title character chloroforming a bartender in order to take the woman's place at a fancy party. The real bartender soon wakes up and manages to make enough noise that she is discovered tied up and gagged in a closet, blowing Catwoman's cover.
Used in Marvel's Spygal one-shot. The title heroine discovers that a French businesswoman has been Bound and Gagged and replaced by an impostor after hearing banging noises and "mmph" sounds coming from a nearby closet.

Film

D.E.B.S.: Amy and the other DEBS are investigating a bank robbery. While passing by a closed door they hear a "Mmmmm" noise from behind it. When they open it they find the bank's employees tied up and gagged, and they "Mmmmm" again.
Frog And Wombat, an independent kid movie about two middle school Kid Detectives who solve a murder, has "Frog" tied and gagged by her school principal, and using her toe to push a button on a walkie-talkie to try to get the attention of her friend "Wombat".
Played unusually in U-571, as the German captive on their sub isn't Banging for Help, he's banging out Morse code: "I am U-571, destroy me."
The ending of The Poseidon Adventure featured this as the way the band of survivors was rescued.

Literature

The Adventures of Archie Reynolds has a hilarious variant of this. Amanda is Bound and Gagged in a crate and banging and "mmmphhing" to get attention. Archie is terrified of the banging, muffling crate for several pages, until he finally realizes "these might be human moans".
Nancy Drew is especially notable in that she (in at least one instance) combines this trope with her knowledge of Morse code.
J. T. Edson character Doc Leroy, temporarily working as a lawman, was coshed and immobilised to prevent his interfering with a planned robbery. (The crooks didn't kill him as Leroy had plenty of the kind of friends you don't want to spend the rest of your life running away from.) He was reduced to banging on the floor of the hotel room he was shut up in, hoping to attract attention.
The main character of What is the What does this when burglars tie him up in his own house. To alert his neighbours, he starts by kicking the floor, then moves on to kicking the door, which makes a good deal more noise.

Live-Action TV

In The Worst Witch in the episode where Miss Cackle gets tied up, gagged and left in the storage closet, when she sees Mildred and Enid flying in through the passage at the top of the closet, she starts making noise.
In The X-Files, a bound and gagged Scully bangs to attract the attention of a police officer when locked in the trunk of Dwayne Barry's car. Unfortunately, this leds to the cop being distracted enough that Dwayne can shoot and kill him.
Similarly, in Dexter's fifth season, Lumin bangs on the side of the car's trunk when it stops at a red light. Unfortunately, the light changes, enabling her captor to speed off before the interested fruit vendor can properly investigate. But he does tell Debra of his suspicions that someone was in the trunk later, leading her to the villain's hideout.

Music Videos

In the video for Street of Dreams by 80s metal band Rainbow, the hero's girlfriend, tied a chair, gagged, and locked in a closet, manages to attract her boyfriend's attention by kicking the door with the toes of her shoes.

Newspaper Comics

Dick Tracy was once held captive during World War II by a villain named Flattop. He hammered out a message with his feet in Morse code, which was heard and correctly transcribed by a young woman upstairs.

Western Animation

Julie Yamamoto in Ben 10: Ultimate Alien does this after being bound, gagged and locked in a closet by Elena in "Revenge of the Swarm". It worked.
Happened in an episode of the original Scooby Doo when Daphne was tied and gagged and had a sheet put on her. Her struggles and her mmmphing through her gag from under the sheet caught the gang's attention, whereas they otherwise might have ignored her.