Think a connection exists between CCD (combustion chamber deposits) and some of the reports of engine noise after switching to M1 or other synthetic oils? The premise is that the deposits tend to get harder when exposed to a synthetic oil vs. dino oils. Based on the gasoline being used, these deposits can begin forming after only 10k to 20k miles. Here is a description of how CCD’s generate engine noise from the Chevron technical section:
Combustion chamber deposit interference (CCDI) and combustion chamber deposit flaking (CCDF) are two other problems which sometimes occur in certain modern engine designs. CCDI is the result of physical contact between deposits on the piston top and cylinder head and is manifested as a loud, metallic banging sound when the engine is cold. CCDI is limited to the engines that have been designed, primarily to reduce emissions, with minimal clearance — 1 millimeter or less — between some areas of the piston top and the cylinder head (squish areas) when the piston is at top dead center. Combustion chamber deposit flaking causes low compression pressures due to improper sealing of the valves. This problem occurs when pieces of CCD flake off and end up lodged between the valve face and the valve seat. Symptoms of CCDF are difficulty in starting and rough running when cold.