Obviously, my comments are genuinely speculative, and I have zero test results to back them up, but here goes, since I believe the noise issue with M1 is very real in some engines.
1. Why would an engine be noisier running Mobil 1?
The fluid properties of the PAO-AN mix magnify existing engine sounds, possibly more so at certain frequency levels.
2. Why could switching to another oil, of the same viscosity, make the noise go away?
Conventional/Dino oils contain VI improvers, friction modifiers, waxy hydrocarbon chains etc. that possibly have a muting affect on the ability of a fluid to transmit noise.
For the same SAE viscosity grade, dino oils will have longer chain hydrocarbons plus long, spidery VI molecules vs a synthetic, thus different fluid properties.
Take a 100 ft length of 2" pipe and fill it alternately with water and chocolate syrup. Tap on one end of the pipe & the tapping noise will be more audible at the other end when the pipe is full of water vs. the syrup.
3. What is the actual noise that users of Mobil 1 are hearing?
Existing mechanical noise within the engine - nothing that actually generates additional wear metal ppm's in UOA spectro analysis.