Unidentified bird call


I live in the Vale of Pickering in North Yorkshire, and my daily walk takes me to a small bridge over the River Dove in a fairly isolated area surrounded by fields, both pasture and arable.

I regularly hear a bird call which sounds like "op". The pitch is medium high, and it is often just a single call, followed by another several minutes later.

I have been unable to see the bird; it is likely to be feeding on the river bank or the adjacent pasture 100-200 m away.

I thought I had tracked it down a couple of days ago, but as I got closer, riverside, and the calls got louder, I startled a flock of rooks, and the "op" sound receded into the distance with the rooks.

Can anyone suggest what bird it may be?