If your mic in and volume out are plugged in close to eachother try moving them to ports that are farther away

I've had speakers plugged in the read with the mic for ages, sometimes plugging in headphones at the front. I've just tried it with my mic in the front instead, to no avail. It happens even when there literally shouldn't be any output sound. I'll try with the speakers unplugged though.

speakers near mics can cause feedback loops

Microphone and rear speakers disconnected. Wireless keyboard turned off and unplugged. No avail, the screech in my headphones is still there (so I'd reason the microphone screech would be too).

Edit: I want to emphasize that it's only when TF2 is active and not tabbed out.

well heck, does the snd_restart command interrupt the noise or not

Check your driver settings, maybe your mic's input is set to a virtual output. What is your soundcard?

!!!

It's hard to notice, but for like 0.2 seconds, the sounds stops.

It's an ASUS P8H61-MX R2.0 (integrated sound). I run linux so I don't think poking around "driver settings" will be particularly useful or easy.

Edit:

i had sound problems in TF2 once, turned out a server had downloaded a bunch off sound files wich made screeching sounds happen,

worth checking folders for any sound files

I know this sounds weird but can you try unplugging your mouse or dimming your monitor brightness?

I removed all sound files from my tf folder. No change. It's not any sound file. It's a quiet, passive screech, like how your old CRT made a high pitched scree when it was on. Except this makes less sense.

OK so none of that helped, I litterally plugged everything but headphones, still heard it, unplugged headphones, headphones go dead (obviously), plugged them in the back, it didn't come back. Plugged everything back in, didn't come back. Plugged headphones back in the front, came back immediately. Tabbed out, goes silence, back to TF2 menu, turns back on. So I've learnt that much, it's definitely a TF2 (or at least a software trigger) and only on the front output port.

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.

As a friend of mine once put it, "stay stange, source engine". :(

Do you have any sound mods (other than hitsound) in your custom folder?

Edit: you mentioned you run linux, I would not be suprised if this is a pulseaudio issue. I'm not sure if you can run tf2 with alsa to test it though.

No, and as previously mentioned, it extends beyond TF2 to all source (and apparently only source) games.

I'm not sure if it's the same thing, but I used to have something similar. I don't remember if it happened in TF2, but I remember it happened to me in Killing Floor. The symptoms are the same as yours and I also have an ASUS motherboard. I would always get this screeching in my headphones and mic, and it would change pitch depending on what I was doing in the game.

My immediate fix was just getting a USB headset which fixed it completely, although I recently got a new soundcard and it seems to have fixed the issue when I use the back ports (the front ones still kind of have it but no where near as bad). From the research I did I assumed it was something to do with EMI.

The cheapest solution to fix the mic would be to get a shitty USB soundcard thing and plug your mic into it (something like this, and it may also help diagnose what it is), although I don't know what that would do to the quality of it and I would definitely not recommend plugging your headphones into it because the sound quality will be shit.

Otherwise if you can't deal with it in the headphones (and I know I couldn't) then I suppose you could either get a USB headset or look for a soundcard with some EMI shielding, or the screeching could just be caused by something completely different idk.

Time to get an USB/external DAC thingumy \o/

And one that supports linux too.

Edit: Thanks btw. "My hardware is super cheap and fucked" (and apparently because it's ASUS?) was the only conclusion I could come up with.

Quick update. I crashed a lot after all this debugging and the sound was so garbled that I had to turn sound off.

I'd forgotten verifying game data was a thing, so I did that and there was a lot that needed fixing. Since then the screeching has gone. \o/

Just need to fix the crickets in the mic now.

Could be something like interference on sound lines when running intensive applications. If tf2 runs at very high fps, I've noticed that causing problems for electronics before.

I literally just posted an update saying the original TF2 sound problem is fixed. The microphone crickets are therefore of course due to interference.