The slow and inexorable march belonging to hundreds of tons of flesh and chitin, completely undeterred by the sheer amount of lead, explosive, and radiation being thrown at them.
"BRING IT FACKERS!!" The sounds of screeching terrors are muffled by gunfire and the glyphids blinded by muzzle flash and exploding balls of fire. Your boots soaked in mucus green ichor, knee deep in a sea of twitching insectoid carcasses. Your voice is hoarse and your mouth is dry from all the yelling you've been doing. The sounds of battle are making your ears deaf and your nose bleed, all you can feel is the throbbing in your head as the endless swarm encroaches upon you.
This feels incredibly threatening. I’m already envisioning an entire swarm of glyphid grunts, a couple of glyphid praetorians, and web/acid spitters backing them up.