Step 1 - The cause of the noise may simply be caused by the unit not being positioned in the window frame properly. If it happens to be loose, the shaking of the unit may cause it to jolt against the frame. In order to check this issue, turn the unit on, press your hand against the window glass, and listen to hear if there is a difference in the noise. If you happen to notice a difference, simply place a weather seal firmly around the unit. You may also try placing shims between the unit and frame.
Step 2 - secure the front panel. Many times the front panel will become loose. This is a common problem and an easy fix; simply tighten the screws surrounding the panel. If this doesn't work, apply some putty in order to tighten the panel.
Step 3 - Sometimes, loose insulations can become stuck to the fan blades. To check, turn the unit off, remove the faceplate, and inspect the fans. If you find any insulation, discard of it and turn the unit back on to listen.
While you have the unit opened up, check the blades to verify certain they are even. An uneven or bent blade can sometimes clash into the interior walls of the unit, causing the noise. Rotate the blades manually. If there is a blade rubbing against the walls, bend it back into place.
Step 4 - Fan motors that require lubrication can also result in a loud hum. Check to see if the motor housing can be opened (many times they can not be). If you are able to obtain access, remove the ports while adding a few drops of SAE 10 oil to each port. Add a few drops along the shaft of the motor and rotate to spread.
Step 5 - Damages coil fins have also been known to cause a humming sound. To fix, remove the cooling coil housing located near the back of the unit. Clean each coil with the brush attachment of a vacuum, and carefully place the teeth of a fin comb between a set of fins that are undamaged to find the correct spacing. Next, slide the comb through the damaged fins to straighten.
There are many AC repair complications that will require professional help. If you cannot fix the unit yourself contact a professional in your area.