Several factors can cause window air conditioners to vibrate and make distracting noises.
1)Check for a loose window pane by pressing lightly along the edges where the glass meets the wooden housing. If the glass moves or shifts under pressure from your fingers, secure it to the frame by caulking both the interior and exterior. Use a caulking gun to lay a bead of caulk along the edges, then press the caulk into place by smoothing it with a moistened fingertip. Allow it to dry for 24 hours to eliminate the window rattle.
2)Locate possible internal causes. Unplug the air conditioner and open the outer casing. Casing designs vary widely among manufacturers and models, so locate the screws or bolts holding yours in place by examining their locations and removing all the likely connectors. Once these are removed, gently pull the casing away from the rest of the unit. If it does not come loose, note where it resists removal and examine that area for additional screws or bolts. The casing should come away from the unit with ease once you find and remove all the bolts.
3)Locate the air compressor inside the window unit. The compressor is a roughly cylindrical component with no apparent exterior moving parts, usually located behind the control panel. The bolts holding the compressor in place are sometimes overtightened by the manufacturer for shipping, which causes the compressor's internal movements to vibrate the entire unit. Turn the bolts counter-clockwise with a wrench to loosen them slightly.
Examine the fan blades. The fan can be thrown off balance by accumulated dirt or grease on one or more of the blades. When unbalanced, the fan causes a vibrating or shaking motion similar to that caused by an unbalanced laundry load on a spin cycle.
4) Blades may also become warped or bent, which also upsets their balance. Clean the fan blades and, if necessary, bend them back toward their proper position to reduce fan vibrations.
Consider having the compressor replaced if loosening the bolts does not significantly diminish vibrations. Do not remove the compressor yourself, because improperly handled refrigerant can cause injury. Instead, take your window unit to a qualified technician for the repair.