I have read a number of posts where people have replaced lifters and rocker arms only to still have the M112 tick/knock noise. Given that it is not a real problem (you have lived with it for over 4 years) I agree with not doing anything until you are confident in its source.
The stethoscope should help confirm if it is coming from farther down in the block, or higher up in the cam area. Any piston-valve contact would damage something in short order, so I do not believe it is that.

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Prost!