I havnt had that happen lately (and hope i dont cause I got a new motor, arm, rubber insert, plastic covering, the whole 9 yards, lol)
anyway, turn on the wipers get out of the car and listen. Also pop the hood and listen by the motor and see if its coming from the motor. You can bend the arm back to examine the springs condition. Another possibility is the plastic peice that hides the nut for the wiper arm. If that plastic peice is put on improperly or loose, it can verywell cause that. Take it off, and snap it back on.
If it does sound like its coming from the motor, get to it and listen/examine. If you do suspect the motor, change it!
I dont know about your luck, but with mine, i will be driving down the highway with heavy rain and the single arm wiper would go out on me, lol. Keep us posted.