Hi and welcome to the site,
It's really interesting that you're seeing this, as I've had exactly the same thing with a Coal Tit (I assume it's a male).
It started last spring, he would stand on a branch outside our hall window and would sing loudly then flutter up against the window. A few weeks ago, I think the same Coal Tit returned to exactly the same bush and window and started doing exactly the same as he had done the year before, it was nice to see him return. Here's a picture of him, I could get really close because he was so absorbed in what he was doing!
My theory is that it is because it is springtime. I think he sees the reflection of himself in the window and thinks it is a rival so trys to attack it and out-do it. It's funny to watch, and I don't think it does the bird too much harm. So basically I think it's these birds being territorial and finding rivals (reflections) to try and scare off. I don't think it does any harm, but to stop him you could probably put a sheet over the window so there aren't any reflections.
Guy