Walkie-Talkie Static

The special sound effect that makes any conversation over Comm Links sound immediately military. A transmission usually starts off with a recognizable click, then follows the actual text, along with audible white noise in the background, then ends with another click. Both sides cannot speak at the same time (also known as simplex operation.)

Goes very well with Danger Deadpan voice, Military Alphabet, Reporting Names, Radio Voice, and Attack Pattern Alpha. However, it doesn't always have to indicate military: Voice with an Internet Connection often sports this sound effect, too, simply for the Rule of Cool.

Amping up the static is a typical way of having a message get Lost in Transmission. See also Fake Static.

This technique is actually a pretty realistic portrayal of modern Real Life voice transmission over frequency modulated radio systems, which for the most part are used by the military, police and emergency forces. The distortion is the reason the Military Alphabet was invented. The click at the start and end is the empty carrier wave while the person is not talking but has the transmit button depressed.

Examples of Walkie-Talkie Static include:

Film

Stormtrooper helmets in Star Wars do this.

Live Action TV

Parodied in The Young Ones episode "Cash". The police recruiting sergeant (who bears a strange resemblance to Mussolini) tells Neil the only qualification he needs to join the force is the ability to imitate radio static when using his walkie-talkie.

Video Games

This is part of what makes Counter-Strike predefined radio messages so awesome.
All Terran units in StarCraft have this in their responses to orders.
Ditto all units in World in Conflict.
Most of the dialogue in Call of Duty 4
Merc in Mirror's Edge.
The Replica in F.E.A.R., who all speak over radio wiith electronically filtered voices. Project Origin actually shows why they only speak with filtered voices, as being clones, their bodies are deformed and their "natural" voices sound like harsh, guttural, inhuman growling.
Most of the dialogue in the Ace Combat games, since it primarily takes place via radio communication between planes.
All unit responses in Homeworld and its sequel, Homeworld 2.
The Star Fox series uses this for the same reason as the Ace Combat example above.

Web Comics

Imitated in this Xkcd strip.

Western Animation

In the Lilo and Stitch episode where they find Clip, when they try to sneak into Myrtle's house, she does the radio click thing with Stitch as though they're talking on walkie talkies (they weren't actually using them, just pretending they were).
In an episode of Bonkers, the titular character refused to stop making this noise, even when he wasn't on the radio.
In Static Shock Virgil and Richie use walkie-talkies they call shockvoxes constantly, from the moment Richie builds them. In the episode where Virgil's sister, Sharron, suspects he's Static, Richie obviously can't hear that Virgil has left and that Sharron is in the room because he's speaking into the shockvox. Sharron hearing Richie call Virgil Static sets off the rest of the episode.
Parodied in SpongeBob where SpongeBob and Patrick pretend to be astronauts talking through radio, eventually just start alternating static noise.

Real Life

Inverted on general-use FM walkie-talkies (FRS/GMRS, PMR 446, etc), where most of them by default include call tones ("Hey you!") and "roger beeps" (indicating end of transmission), but due to the way the squelch functions on such walkie-talkies work, the user never hears the static unless the walkie-talkie is put in monitor mode.
Many professional and ham radios have an adjustable squelch control to keep constant static from being annoying. Weak signals may not be able to break through the squelch, so the Lost in Transmission trope can (and often does) occur.