I posted about this in another thread, but in my experience I have found Panasonic to be one of the quietest running window air conditioners you can buy. They've also got a very nice look to them, and are very energy efficient.
My parents went through quite a few brands of air conditioners before discovering Panasonic, and now they've got three of them. Their Panasonic units run about as quiet, if not quieter than a small fan set to low. Interestingly, they've got a ~3,000 BTU, a ~5,500 BTU, and a ~13,000 BTU unit, and out of the three of them, the ~5,500 BTU is the quietest. The 3,000 BTU unit is also over 10 years old, so that might have something to do with it.
Anyway, I can highly recommended a Panasonic for a good window air conditioner!