the sound of a physics prop stuck in a wall somewhere.

where?
who knows!
all that you do know is it wont stop.

the fart get suddenly wet 

The red skeleton

Footsteps. Very loud footsteps.

im gonna have to say a more abstract one, a distant door opening.  

all the things happening nearby that startle someone like footsteps around you will spook you in the moment but then you turn around and can confirm nothings around you and it was a spook. the game also kinda likes doing things like that with noises around you that dont mean anything so footsteps would spook you but you kinda have context for them not meaning anything.

the game might open a door near you sometimes, but it never arbitrarily has a distant one open.  when you hear that you know something opened it, and it wasnt you, and it probably wasnt nothing since the game doesnt do that distantly. youll know somethings here but you wont know what.