It came from an old thread but this bright idea really helped me
 

I tried the "bending it straight" technique and it really works. No need to buy new blades nor put anything on the windshield. Visual inspection of the arms left me aghast looking how skewed it was. Since it was twisted it keeps the blades from standing perpendicular to the windshield, thus the chatter. It is now as quiet as it can be. No more chatter at totally no cost. Hooooray!!!

This is a simple DIY and a great information. For those who are using the search engine, keep on using it. You might just find the answer to your problems with your Mercs.

When I check my wiper arms I remove the blade assembly and use a stick inserted into the hook on the end of the arm. It is about two feet long and it becomes extremely obvious if the arm is bent and needs to be twisted back.

If it does need a tweak, I use a small crescent wrench to twist with while holding the part connected to the motor arm.

Haasman

I tried using a long screwdriver in the hook of the wiper arm to make it strainght. When the arm sits on the windshield. It is resing on its edge, so it is bent a little. I grasped the wiper arm with pliers and I uses the screwdriver in the hook to unbend the arm. So far no luck, the wiper arms just keeps on sprining back into its orig shape. I dont want to put too much force on the arm yet otherwise I see no results.

Dear 300D,

Don't be afraid to put more pressure. I did mine using pliers while holding tightly on the base of the wiper arm. Just be careful not to overdo it. I would suggest that you give it a twist at a time while doing a visual inspection each time until you get it straight and not give it one big pressure that might destroy the whole arm. As the saying goes "the world was not created in 1 day".

I read through your post entitled W124 wiper. Before I read about the solution mentioned above, I'd like to point out that I've also gone through this route as far back as Y2002:
1. had my wiper disassebled by a competent mercedes technnician in early 2003. He cleaned and lubricated my wiper.
2. Bought new wiper blade assembly
3. Placed RainX several times

All of these did not solve the annoyingly noisy wiper.

I don't know if the problem you have is the same as mine, but you mentioned about your wiper arm resting on its edge (which edge by the way?).

One good reason to believe that your noise is emanating from the wiper blade is when the blade itself is not resting perpendicularly straight on your windshield. This is the time when the blade needs to be replaced or the arm is twisted and needs some straightening.

I hope you keep us posted.

Dear E300D,

Sorry for the inadvertence, I used an incorrect call sign.

Regards,

124Addict

No problem W124 Addict. I like your call sign because I think im getting addicted to W124 cars!

Good tip! I'll check mine for this.

Although the only skittish wiper is on my W124. Periodic cleaning of the blade usually cures that...