Resurrecting a 10 year-old thread.

I replaced my drivers-side check strap today. About 7 months ago, I had the same noise and two loose bolts (above and below the oval hole where the strap emerges from) described in the original post, and tightened them.

During the time they were loose, the metal feet/tabs those bolts screw into deformed. After tightening them, the deformation changed and the constant opening/closing of the door caused them to flex repeatedly and break off. At that point, only a single bolt was connecting the strap to the chassis; and the door would make two LOUD cracking sounds whenever it was opened.

It's a good bet that if your check strap bolts have been loose for some time, they are likely close to failure. Thankfully, it is one of the easiest, fastest and least costly repairs you can can do. ($25 for new strap and 20 minutes to replace.)

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2008 E350 4matic / Black/Anthracite

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Gone but not Forgotten:
2001 E430 4matic, 206,xxx miles, Black/Charcoal
1995 E320, 252,xxx miles, Black/Grey
1989 260E, 223,00 miles, Black/Black