Random dumb tech story: For months my computer microphone has had a lot of loud static on it. Every time I say something in a game like Left 4 Dead 2, people complain and tell me not to talk again.
I finally got around to investigating the issue, which involved unplugging things, looking at the sound controls, etc. I was bewildered to find that the computer was recording noise even when the mic was unplugged. Then I realize: I have a USB webcam that I rarely use. It's plugged into the back, it has its own built in mic... and it's hanging behind my desk RIGHT NEXT TO THE FAN.
Everything's crystal clear now.