River warbler – Locustella fluviatilis

This is a largish warbler. The adult has an unstreaked grey-brown back, whitish grey underparts, and a darker undertail, which has white feather tips giving a contrasting pattern. The sexes are identical, as with most warblers, but young birds are yellower below. Like most warblers, it is insectivorous.

Some birds can show reduced dark markings on the undertail-coverts (caused by more extensive than usual white tips) and thus are closer in appearance to Savi’s warbler than typical birds. However, they typically still have a streaked breast and more olive colouration on the upperparts. This is a skulky species which is very difficult to see except sometimes when singing. It creeps through grass and low foliage.

The song is a monotonous mechanical insect-like reeling, often given at dusk. It is similar to the song of other species in the group, but has more of a sewing machine quality, and may be produced for long periods.

It sounds like this
Recording by Graham Catley from Xeno canto