No way I could wait that long!
Heard a noise, got my cheap parts store stethoscope, located it and pulled the valve cover in about ten minutes. This is what I found. Borrowed an adjuster and had it back on the road, no problems for many, many miles since.
Why sit around wondering what it might be, asking other people and torturing yourself? Take 20 minutes and go pull the covers. You'll feel better just knowing.
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