Strange noise coming from the reed on the pond.
That day we have passed close to few ponds, however only two of them made that strange noise.
Sound seemed to come from all over the pond. Walking up to the edge of the pond, close to the reed, strongly amplified the noise. Coughing shouting and tossing a large stone into the reed/water didn't seem to interrupt the sound even a bit. No animals were visible, no movement in the reed etc.
The sound seems to peak at 400 Hz and it could have been heard from 100-200 meters.
It's spring here, so we suppose it might have been a mating season of some frogs or insects.
EDIT: It seems those were the sounds made by fire-bellied toad (bombina bombina).