If the car is a few years old, there's a strong possibility that the
bearings at the base of the wiper blades are dry and/or seizing. It's
pretty easy to tell. Lift your wiper blades off the glass and turn the
wipers on. Watch the housings where the blades attach to the car. If the
housings twist as the blades move, the bearings are seizing and in need
of lubrication. Park the wipers, then take off the wiper arms. Remove
the rubber seals covering the wiper arm bearings and work some oil
(don't use thin spray lubes, they don't last long enough) in between the
shafts and the housings, then turn the wipers on. Continue to add oil as
it works its way into the bearings. When the shafts turn freely in the
housings, you're all set and you can reinstall the wiper arms. You'll
probably hear a difference in the wiper motor sound when the bearings
loosen up.