My roommate Veronica recently acquired a window air conditioner for her bedroom. She loves it, but she's a light sleeper, and before she bought the device, she was worried that the noise would keep her awake at night. I told her that these appliances have gotten much quieter in recent years, but that I would check online to see if there is any way to reduce the noise that these machines make.
It turns out that there is a lot of information out there about this topic. Everything that I read said that the most common reason for excessive noise from an air conditioner has to do with improper installation. For example, most of the articles I read told me I should make sure that the side panels are fully extended, and that the AC fits snugly in the window. Not only will this help it run more efficiently, they indicated, but it will block out noise.
But occasionally, the rumbling noise that an air conditioner typically makes will change, and it will sound like it is dragging something along. If this happens to your model, check the fan blades. Sometimes a blade breaks, or something becomes caught in the fan, which will cause your AC to make a strange noise. Simply remove the front of the appliance and look inside to see if this is the problem.
But even when they're running great, every AC does make some noise. If you really can't stand the sounds your device is making, go to the hardware store for some soundproofing material. You can put extra insulation around the window, or pad the walls with cork tile. Anything you do can help to block out excess noise.
Veronica was extra careful in installing her air conditioner, making sure that it was properly screwed in to the window frame, and that the panels were fully extended. She happily reported to me that the noise hasn't bothered her at all so far!