Knocking on M157

[Finestey]

Good day.
This little clunk or bang came out of nowhere and was really bothersome.
I've checked the oil filter for flakes, replaced the spark plugs with the correct torque, used new MB 5w40 oil, and used the borescope to ensure that the sleeves are OK. However, still nothing. It seems like the noise went away the day after the oil change, but it came back the next day.
Not audible at idle, but detectable between 1000 and 1500 rpm when the engine warms up, and then practically goes away when the engine becomes hot.
I ran the Xentry diagnostic last night and everything came back clear: no misfire, no errors, no stored codes, etc. The engine pulls nicely, but I'm afraid to push it to its limits because of this.
I tried using a statoscope on my own, but the noises weren't suspicious—just like the ones I mentioned before—and the tapping wasn't happening while the heads were at rest.

Where can I find out what I need to know before I waste money on guesswork and garage advice?

I came across a discussion on a similar subject, but the tone changed abruptly, and it is unclear if anyone has resolved the issue.
Best regards

[MagicPod]

Can you hear the small knock coming from the area around the offside wheel arch? Mine was most visible there, and after I mentioned it to the mechanic at my neighbourhood Mercedes dealership, he assured me it was caused by the secondary air pump and that I shouldn't be worried.

It was a minor problem that never got worse on either of my E63s powered by M157 engines.

[Finestey]

No, on idle it's completely silent from both sides; it only starts to cluck somewhat under load at low RPMs, about 1000-1500. Many people have suggested high pressure pumps, but I haven't been able to prove them wrong.