Vibration is the main cause that makes air conditioners noisy. The problem is that vibrations can be caused by many components, so in order to quiet the unit, you will need to make a vigorous inspection.
Checking the insulation
The condenser is usually insulated.
Sometimes noises come from the unit because of it’s little fixation. Turn the unit on and if you push it to the wall and it stops the noise, then you should better fix it to the wall. The problem can also be solved by adding some wood shims in the gaps between the unit and the wall. You can also use putty.
Most of the times, the cause of the noises are loose louvers. To see if this is the problem, turn on the unit and hold each louver to see if the noise disappears. A tighter fit for loose louvers can be accomplished by changing the angle. Also Styrofoam solves the problem.
Another problem that occurs very often is that of fan blades. Most of the times the fan blades get deteriorated and thus cause vibration. The wear can appear if there are extreme weather changes, or because of accidents. Inspect the fan blades and see if they are all the same. If one is bent, straighten it and then turn the unit back on to see if the problem is solved.