This has been a problem since the game launched but rare enough that you hardly notice it. However the addition of the shock troop mutator has made it much more common and I feel that something should be done to prevent it.
Basically the problem is that poxbusters spawning inside of a door next to the players do not make any sound when they emerge and attack. ALL of the sounds that the burster normally makes such as the ticking and the final scream as they jump and detonate themselves are missing. Often the first and last indication the players get that anything is happening is when the poxburster explodes.
There should be a minimum distance from the players where poxbursters (and any other specials) cannot spawn, because if they don’t make any noise there is no possible counterplay against them.