Here is what a typical owner's manual will advise:

1) Unplug the unit and remove it from the window (beware, these are heavy!). Put it where you can work on it outdoors.

2) Remove the grille and filter and unscrew the metal case. Be careful not to damage the coil's fins.

3) Wash out the filter or replace it with a new, inexpensive filter--cut-to-size foam filter fabric is available at most appliance stores.

4) Using a vacuum with a soft brush attachment, clean the inside coil's fins.

5) Then, from the fan side, spray water back through the fins (protect the wiring and motor with plastic).

6) Clean it up with a rag, making sure all drains that allow condensed water to drip away from the unit are open. Allow it to dry thoroughly.

7) While you have the unit apart, lubricate the motor according to your owner's manual instructions.

8) Then reassemble and reinstall the unit.

If that doesn't do it, clean the evaporator and condenser coils.