Hi Terry and welcome to the forum. Your problem is not an uncommon one. In fact the phenomenon runs to a 6-page thread over on MB World. Your dealer can polish the windscreen for you using a specific MB product and can also check the wiper to glass alignment using a special gauge. Others have found this problem arises when they refill their washer bottles with something other than MB Summmerfit/Winterfit fluid or apply a hydrophobic product like Rain-X to the screen so I suggest you don't go down that road in the future. Here's that more detailed thread on the subject:
Anyone know of aftermarket wiper blades that will fit rather unique MB mounting system ?
My wipers chatter across glass, dealer cleaned glass, no better. I noted that actual blade cross section has a rather short
Amount of blade rubber, maybe not enough to allow rubber to flex back and forth as wiper changes direction, as compared to other brands where the flexing part of blade is much taller.
Same problem in New Zealand - New (2018) GLC 43 AMG Coupe. Wipers juddering first time in the rain. Been back 3 times now and they have done all the "special tool, polish screen, rest angles etc etc. Marginally better but not right. Pity because I absolutely love the car, MB just needs to get this problem sorted and do a recall.