The mutant from Quake 2 makes a loud noise when he sees you and when he dies. He is appearing in place of a demon and I don't think a low tier monster should make a loud noise like for example a high-classed enemy. I know high class monsters should have this behavior but not low-tier monsters, right?
This bug is minor and low priority but I think it gets annoying when in a map with a lot of "demons" and this monster sound can get irritating.
EDIT: I think this monster was previously a Hell Knight class monster and that's why he does make this noise, as he kept some previous changes.