The K8400 has a small ventilator(fan) at the bottom of its case.
The manual states, that its blowing direction is crucial and should point downward under the casing.
It is covered by a small grill. All was mounted according to the manual.
When switching on for the first time, I heard a loud mecanical noise, and pulled the plug immediately.
The source of the noise was found out quickly:
That small fan.
Dismounting the fan grill, untightening the screws: Noise stays the same.
This small fan at that position makes simply no sense - only noise.
I finally found out, that it makes only that noise, when mounted horizontally with blowing direction downward.
Since its purpose (originally?) is to cool the stepper motor drivers and not to make noise:
Mounted it - blowing direction upward - directly under the xyz stepper motor drivers.
Kept in place with four cable binders in between the cable trees of the mainboard.
Result: No more noise, cool stepper motor drivers. 
Btw: The holes in the base plate above the switching power supply are quite senseless, too…