Zombie Panic Source dropped this grimy, oppressive soundtrack in 2008 and it nails the Half-Life 2 mod atmosphere perfectly. Every track is designed to make you paranoid. "Chapel of Unrest" has these droning textures that just sit in the back of your skull while you're checking corners.
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"Sewers" goes for maximum claustrophobia with low rumbles and distant metallic sounds that keep you on edge. Then "Horde" kicks the tempo up when everything falls apart. The theme song has this scrappy, indie-horror charm to it.
"House of Whispers" does that slow-burn tension thing where the silence is almost worse than the noise. For a Source mod soundtrack, the production quality is honestly impressive. If you ever played zombie survival mods at 3AM with your friends, this is the exact vibe.