I have the same INA pulley on my SDL, put an end to the belt flutter right now.

The spring and shock should be replaced together as a pair. When the shock goes, the spring can fail sometime later on from stress. The spring keeps tension on the belt, the shock dampens the oscillation. When the shock fails, you get the rapid fluttering and rough rattle that you show in your video.

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Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]