Spaz sometimes screams at me! What's that about?
If you send a tweet of exactly 140 characters, Twitter makes a certain sound. The equivalent in Spaz is the "Wilhelm!" noise – the scream. If you don't like it you should be able to just swap in a new mp3 called "wilhelm.mp3" – but it seems AIR doesn't let you. And down the road, it'll be easier to swap in new sounds.
You can disable this feature by unchecking "Wilhelm can scream" under Sounds in the Preferences section.