Secondary Air Injection pump... it's an emissions reducing device and the bearings in them are known to fail causing lots of noise and eventually will overheat and seize, throwing the belt.

If your state doesn't require emissions tests then just throw it out.

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Current cars:
2000 ML55 AMG, 174k miles
2003 C240 T-Modell, 202k miles
1995 S320, 207k Miles