From here, you should be able to find a sound. Then, place an ambient_generic entity open the properties by selecting it and pressing alt+enter, then click on the sound property, and click browse.
If nothing else, you can use the Hammer Editor to look for sounds open the Source SDK from steam, and select the Half Life 2 configuration. gcf files, this isn't an easy task I recommend using an in game browser to find these sounds. You have to search through all the sound files, and since most of them are in the. The hardest part of adding a sound effect is finding the sound itself.
