Gotta say. I ran as fast as I could when that thing popped up with it's scary ass roar.
Man, that screech gave me flashbacks to the revenant's spot sound effect in Doom 1/2... it was the stuff of nightmares.
I've also noticed that some enemies use stock sound effects that were also used in Doom, such as the imps' and either the pinkies' or barons' (don't remember which). Also, it sounded to me like grexes did a little dog whince SFX whenever you toppled them, which was funny as hell.