Since my 95K mile bike does something similar.......I'm not going to attempt to answer your question. I've lived with my "sound" for some time and try not to let it get to me.
Your post reminds me of a supposedly true story that goes as follows:
A certain rather particular member of a British flying club came back after a flight one day and wrote the following problem entry in the log book: "Engine making unfamiliar noise...."
The next day he read the following corrective action entry from the mechanic in the log book..."Engine run for 45 minutes. Noise now familiar."
It is also known as the "K" rattle. The crank has a gear that drives the output shaft gear that drives the alternator gear. All these move a bit at idle and under no load. That is the noise you hear. They all do it some are just noisier than others. Normal operation.
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