I had the same problem on my C class. The cause is a dry slide on the mechanism at the bottom of the wiper arm.
Carefully prise off the plastic coverat the base of the arm, I cannot remember exactly where the best place to prise it off is but with care it does come off without damage.
You will see a round bar which slides in two bearings, lubricate this with some oil, I used oil with Molyslip added. Check it works ok and replace the plastic cover.
Mine has been fine for 2 years now.
Kind regards, Guy.