I am by no means a professional 'sound guy' but I do enjoy playing my music and movies without any outside noises. It sounds like you are suffering from a similar situation I deal with. Often it's not about preventing 100% sound from coming in, but preventing it from reflecting and propagating.
Some common anti-echo techniques I employ are:
- hang framed Canvases (cheap oil paintings or blank canvases from a craft store)
- Rugs/Carpets (thicker the better)
- Foam Panels hung on doors
I always imagine if every hard surface is mirror plated and you shined a light from where the noise comes from. It can be reflected off just about anything. The softer and less dense, the less reflective.
If it comes right down to it, and the window truly is the source of problems, an inexpensive project can be making a frame for a single piece of poly-carbonate to mount/hang over your window. It essentially would make a crude "triple pane" window.
I've never had to go this far for noise, but I have for apartments with drafty windows and it was noticeably quieter.
Good luck and let us know what works for you.