Samson makes decent microphones. The meteor is a Cardioid pattern, so it will mainly pick up sound from the direction it is facing, but that doesn't mean it won't pick up anything else.
The better the microphone, the more sound it picks up. Sometimes there are filters built in to protect from low frequency transients, like mic bumps, but generally it will pick up everything.
Some possibilities
- Maybe your fan is louder or closer than these others you are hearing on Twitch. Sometimes a few inches matters. Some video cards are terribly loud.
- Some chat software includes its own stuff for echo cancellation, noise reduction, and feedback protection. I don't use Mumble or Teamspeak, but that may account for why they don't annoy you but raw recording does.
- Too much gain; make sure it's just adjusted to the point where you can hear yourself talking with your headphones on. Even if the microphone doesn't come with software, Windows will provide basic settings from the right click menu on the speaker taskbar icon, and that should include gain.
- If you are doing home recording you will want to eliminate sources of noise. That means isolating yourself from the computer as much as possible, using less CPU/GPU or otherwise quieting the fan. If you can't do that, you need to use a noise gate that will only let sounds of a certain volume through (though this can't completely fix your issue.) You can also remove noise from your recordings with plugins that sample some of the background noise, invert it, and apply it to places in the recording that are quiet. This also is not perfect.
- Headset mics are a lot better at keeping noise at bay because they are near field and don't need to be very sensitive to pick up sound that is an inch or two away.
- Your house might be louder than you think. You get used to the noise floor in your home pretty quickly.
- If it were hum, it could be a ground loop, but that isn't very likely with a USB microphone, and you said fan noise.
