Ugh, I hate that I have no way to record it for a better report.
But I have noticed it multiple times when I engage a Gun Hulk in Junkertown,
as soon as the battle screen opens (as its walking in, or ‘activating’ and standing up)
often times I will get a loud sound effect (I believe it sounds like the Puncture Armor attack it uses),
it is very loud (much louder than the fight itself).
It has happened multiple times, but I will try to get a recording today.