I hear a slow, low groan, a low pitch vibration, from the rear when traveling at very-very slow speeds, typically in crawling traffic (I live in metro New York City). It seems to cycle with wheel speed. It goes away when speeds reach a few miles per hour. I experienced the same groan on the 1985 500SEL, and 1986 560SEL. Searching for "groan" threads refer to wheel bearing failure. In previous cars I have had wheel bearing failure and it sounded very different, and was typically at high speeds. I am no expert but it sounds to me like drum brake dragging? Is the emergency brake a separate system? Is it a drum system? Resetting and re-releasing the emergency brake had no apparent effect.
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