I just opened up CS:S, L4D2, HL2, and Gmod, some with and without opening vids. They all had a screech, and they all seemed to have their own screech, which is fucking weird. So it seems it's source games.
Edit: High/Low audio quality in-game settings makes no difference.
Edit: It doesn't go away when I change in-game volume (in the options menu) to 0. It does go away in the freeze-moment between moving from the main menu to a loading screen,
Edit: What the fuck. Muting it from outside the game, using cinnamon's audio controller, doesn't make the screech go away, Yet the in-game command snd_restart makes it pause for a moment.
??????
Edit: Muted my whole system, no output audio from any applications. Screech still comes back when I tab back into TF2.
Edit: That goes for all source games. The screech (their own special snowflake screech) does not die when the whole PC is muted.
Edit: I've unplugged the SATA and power cabled connected to the HDD. Only the SSD left plugged in. I also removed the mobo-buzzer. Still there. I am fast running out of ideas, and I don't really want to rock up a linux chat room and ask how much damage I could do by live unplugging the SSD.