I had a sound just like that on my SD, the compressor/A/C don't work anymore, but it still had a belt (in poor shape) and whenever the engine was at around 1,200-1,300 rpm I'd get this rattling noise, it happens to be the rpm at 25-30mph so it was quite annoying in the city. I checked and checked and finally figured out it was the clutch plate on the front of the A/C compressor vibrating at that particular speed. I cut the belt off, and the sound is gone!! Someday I hope to have A/C......thats a day a looong ways away.

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