aaah... I listened all over the top end of the engine and belt-driven components at operating temperature but cannot isolate the sound that I've described. But I felt I was closer to this knocking sound when I put my head just underneath the driver side wheel well... Since this noise is hard to hear from the outside but easier to hear from inside the car with windows/doors shut, I am led to believe that it's something that mounts closer to the cabin area itself or maybe underneath it - this leads me to suspect the tranny... Can tranny make noises like that? (It's wet in the parking lot, so I couldn't crawl under to listen closer...)

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84 300SD - bought in April '06 with 237,000 mi