Great reed warbler  – Acrocephalus arundinaceus 
is a Eurasian passerine in the genus Acrocephalus. It used to be placed in the Old World warbler assemblage, but is now recognized as part of the marsh and tree-warbler family (Acrocephalidae). A. arundinaceus are medium-sized birds and are the largest of the European warblers. They breed throughout mainland Europe and Asia and migrate to sub-Saharan Africa in the winter. Great reed warblers favor reed beds as their habitat during breeding months, while living in reed beds, bush thickets, rice fields, and forest clearings during the winter. This passerine bird is found in large reed beds, often with some bushes. On their breeding grounds, they are territorial. In their winter quarters, they are frequently found in large groups, and may occupy a reed bed to the exclusion of other birds. Like most warblers, it is insectivorous, but it will take other prey items of small size, including vertebrates such as tadpoles.

It sounds like this
Recording by Mikael Litsgård from Xeno canto