I was recently at night in a public park in Karlsruhe, Germany. I've heard many (at least 10, probably many more - it's hard to tell) animals in the trees making noises similar to that of a rubber duck which is pressed.
What could that be?
It is most likely a Tawny Owl (Strix aluco), or Waldkauz in German, one of the most common owl species in Central Europe. These owls have many different calls, and you can hear them often here in Karlsruhe at night. Particularly their "kewick" call sounds similar to squeezing a rubber duck. It is usally emitted by female individuals during the mating season, but also males can produce it. Juveniles have a similar call as well. But most of the time - as far as I have noticed - you only hear their more common "hoo-hoo" calls (listen here or here).
Photo of a Tawny Owl: