Until we can afford to have central a/c installed in our home, my wife and I have to get by with various window a/c’s. It’s better than nothing, of course, but they can be truly noisy! First there’s the hum of the fan under ordinary operation. Then, when the compressor comes on, there’s a thunking noise. Until I got used to it, every time the compressor in our dining room switched on, the thunk would wake me up! And every so often, I have had to adjust the fitting in the window because a rattling noise occurs. Sometimes, I’ll thumb through the owner’s manual & stumble upon an item that I overlooked when installing the system. For one thing, one should pay special attention to any gaskets, as poor installation can lead to a rattling noise. Also, any vibrating noises might be linked to shoddy tightening of the mounting screws. Rattling may result from the glass panels in the window, which can be eliminated by applying window putty. Sometimes noises come from inside the unit. The manual advised that I check the mounting parts for official tightening, or that I slightly reposition the unit itself. A neighbor of mine who works in the Heating & A/C Industry once mentioned that if the fan blades get dirty, it’ll cause them to get out of balance. Once this occurs, it wears out the bearings. In the end, I realize that some of my existing window a/c’s are getting older. This alone explains why my units are noisier than the newer models. All of these things remind me to keep saving my pennies so that one day soon I can afford central a/c for my home.