That's the noise a bird makes flying into my Thermopane windows. They been doing it more and more. Just this morning I watched a blue jay perched on my porch railing. The jay spread his wings and took off, gathered speed and whap! Right into the window. He must have survived, I didn't find him lying dead in the driveway.
And it ain't like that window wash commercial "Our Stuff makes glass so clean it seems to disappear." My windows have a goodly selection of bug smears, condensed wood smoke and they all need cleaning.