Fortunately, I've diagnosed similar stuff so often, it's becoming second nature. For me, it's usually the pedals or cleats. Easy to check the pedals ... just hold on to them and wiggle them. If there is any play, that is at least part of the problem.
I had a clicking sound that drove me nuts on the 508. The source? The front wheel (Easton EA-90 SLX). It has a little mechanism for pre-loading the front bearings ... something that needs adjustment every 100 miles or so. Once adjusted, the sound disappeared.
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Proud parent of a happy inner child ...