The sound of the 30mm cannon of the A-10 being fired is almost supernatural. It sounds like the sky is being torn in two:
You hear the explosive rounds impacting first, followed by the demonic snarl of the gun's report.
The 20mm cannon on the front of a AH-1W SuperCobra sounds menacing as well; the first time I hear one I thought it was some great Norse war horn or something. And once while watching the stars over Ramadi while enjoying a tasty cigarette I saw a bright red thunderbolt looking monster grow in the sky. It scared me for a second because I had no earthly idea what it was until I heard that tearing sound, and then I heard the twin turbine engines from a F/A-18E SuperHornet. Would not want one of those shooting at me.
Check out this thread at Military Photos.net for some more cool war footage.