"Acoustical Curtains" won't help if you are getting rattles from the windows, and won't hold any low end sound in the room. It will help a little with the reflectiveness of the room to help reduce some echo. Acoustical curtains need to be pretty thick, but not so thick or tight of a weave that they actually block 100% of the sound. That would turn the curtain into a diffuser of sorts and scatter the sound around. You want something that will let some of the sound filter through the fabric so that it can get trapped in between a couple layers (or inner/outer layers of the same) fabric.
Just know that whatever you do will probably be better than doing nothing. While you might not get stellar results, as long as you get results that you can live with that is OK too.
Good luck!
