I’m betting two milspec turrets, which are M4s firing 5.56 NATO and pretty capable of hurting you through armor. (I designed them to counter folks who would wear survivor armor and complain that they could safely ignore 9mm turrets.)
Their beep-beep is comparatively quiet and the far one shot through the other (aiming at you), but repeatedly hit and eventually destroyed the near turret. Cue explosion, and apparently structural damage.
(There currently aren’t any tankbots in the bunker, but I’ve considered parking some in the structure, activate if disturbed or someone tries to break in.)