First off, love your work!!! Thank you for bringing this experience to us.
Just want to mention that the zombie window on Kino Der Toten (Facing stairs, it is bottom floor left window) has an invisible barrier that stops my bullets.
***Edit*** There is also a super bassy audio playing near Staminup, think it is supposed to be the generator or something, but it is drowning out grenades and such, quite uncomfortably loud, unsure if it is intentional or not.