F.Y.I. the secondary tensioner shim P/N is 40095-01K and I know RustyG among many others here on the site has experience with setting that secondary cam chains tension with the tool P/N HD-45334 and adding shims to get rid of the start-up rattle. What doesn't make sense is how someone could drop that shim inside the engine as its on the outside cap end of the tensioner, so it may be something else ? Someone here will know. If you still have the oil filter I would open it up ( a sharp chisel around the base will do it if you don't have a cutter, and wear thick gloves ) and inspect for any small non metallic debris like aluminum. Hopefully you lucked out. Recheck at next oil change, shorter interval though.