It could be a lifter(s) going dry, low oil pressure, clogged oil filter--the list goes on. Have you checked for engine codes? If it is a lifter issue the engine would likely run rough. I don't use oil additives, especially those the are supposed to seal leaks and I never vary from recommended oil wt. range. The reasons I don't are sealers can clog something up and flushes can break a harmless deposit loose the gets lodged somewhere that causes a problem. That said, the flush was worth a try to get the sealer out. In your case I suspect the heavier oil is contributing to your problem.

It won't hurt to try a new chain tensioner but be sure you get the revised on that is longer than the original one. I have nothing for verification but it seems like the longer tensioner would extend the life of the chain guides--keeping the slack out of the chain for a greater number of miles. I changed mine just for that reason.

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X5 4.6 2002 Black Sap, Black interior. 2013 X5M Melbourne Red, Bamboo interior
Dallas