Usually the checkstraps on w124's have no adjustment to them other than lining them up when mounting. They work or they fail. Generally creaking means they are on their way out. At $12 each, just replace them at the first sign of trouble. You can lube the channels on which the spring-loaded ball sockets ride and see if that helps, but usually when the noise or stiffness begins they are failing, and need to be replaced.

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Stable Mates:
1987 300TD 310K mi (Hans)
2008 Jeep Grand Cherokee OM642 165k mi (Benzrokee)