Without actually taking it apart, it is hard to say what it could be. If the top part of the engine is starving for oil, it will make these sounds. The oil galleries on the top part of the engine could be clogged. If the lubrication is not there the parts begin to wear out quick. I am not a big fan of synthetic oils and I don't agree with adding synthetic oil to increase oil change intervals. I have used both and the additives which enhance the oil break down just as quickly when heat and friction is applied. I Michigan, like New York they put down a lot of sand & salt and that stuff gets sucked up in the air intake and small particles find there way down to the crankcase causing premature wear. I would recommend removing the valve cover to inspect the parts and oil holes to see what's going on.
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