Great advice diseasel300

Make sure it is the tensioner / pulley with a stethoscope or long screwdriver. My alternator bearing was going and it didn’t start making a racket until the AC was on. Whatever it is, it should be fixed ASAP. If the tensioner goes while running it can cause all kinds of other damage.

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07 E350 4Matic wagon, 06 S430 4Matic, 99 SLK 230
retired W210 (x3), W124, W126