It mainly happens while I play Gmod, However it has happened other times to. the buzzing sound only plays in my ear buds though.
It might be your earbuds. I plugged my earbuds in my school's computer which the audio port was a little more wider than the headphone's wire. So it makes a buzzing or clicking noise overtime I move or just sit there. If I switch to my laptop, it doesn't make any noise anymore. If the problem is still there, just look up on Youtube or something and find some info from there.