What is making that rattling sound when I go over 20 mph?

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My car just started making a repetitive rattling/clanking noise when I drive over 20 mph. The Mechanic thinks it is a transmission-related issue, but cannot pinpoint the cause. No error codes or dash messages appear. Manual Transmission. 44K Miles. I am wondering if it is this: http://repairpal.com/intermittent-ticking-noise-from-valve-train-532 Any help is greatly appreciated.