The accursed pacman sounds
support.mozilla.com - Pacman sounds play in background in Firefox
Turns out I'm in this group - I'm running Iceweasel with the CoolPreviews add-on. And the incessant siren was so annoying it's almost comical. I turned off previews for the time being but I must tell you the whole ordeal was rather bizarre.
Bizarre in the sense that this seemingly random chain of events came together the way it did. Google, for the first time, has replaced its title with a real flash version of pacman - complete with sound. And CoolPreviews just happens to run in the background all the time, checking all the links on google for a preview (or something like that, I can't really think straight anymore to be honest). And the result is that Firefox has a bizarre siren noise playing ALL THE TIME, a la malware.
And the flash game running in the background took up about 20% of my CPU time (granted, it's a single core 32-bit processor from 2004. But still, 20%?)