Salty first this is HL2...
Second Hunter is correct...
To do what you desire you place an ambient anywhere, go to properties and untick in the 'flags' section 'Does not loop'. Although I doubt you want a sound like owned owned to play over and over...I am assuming you want an ambient sound right? If you click on a sound in hammer is will play and most looped sounds will have the word looped on it e.g.
sea-waves-loop.mp3