Take the valve cover off and take a good look for any broken valve springs. I had a broken spring that I found during a head gasket change..not sure how long it was like that, but my "tick" I had stopped so it must have been that.

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2016 Monsoon Gray Audi Allroad - 21k
2008 Black Mercedes E350 4Matic Sport - 131k
2014 Jeep Wranger Unlimited Sahara - 62k
2003 Gray Mercedes ML350 - 122k