I have a 2000 SL500 that I just purchased with 20K miles. Both doors made a sound that was very annoying for what I believe is uncalled for in a car of this caliber so I had both Door Check Straps replaced by an Indy for $225 labor + $53 parts = $278.
As far as I am concerned it was well worth the cost. Both doors are like new. It appears that a ball bearing like part in the door stay goes bad so no amount of lubricant will fix it. In fact I tried WD40 and it made it worse. The Indy showed me the used part and it had plenty of grease.
Please read
http://www.benzworld.org/forums/r129...tallation.html and
DIY-Replacing Door Checks