Link to the most famous example for the curious.

Though I prefer the Zombine (infected Combine soldier) sounds. It's a mix of failing hardware and them desperately trying to alert the others that they're infected. It's also 90% made up of edited voice lines from the basic Combine soldier, which is some brilliant asset reuse that makes them feel even more unnaturally robotic.

Edit: and for completeness, the infected HEV suit from Black Mesa.