Thanks for your comments...
Today I went through my Naniwa progression.
Almost no sound, especially at 8k and 12k...

Must be the shapton glass stones.

Anyway, I will try to find out more where the sounds come from... Also the feeling.
Feels like a grain of sand under the blade.
Water rinsing doesn't help (blade rinsed too).
It's like there is still material that needs to be abraded but takes forever since the grit is too fine...
I will also try to step back to the previous stone and then see if it's still there when returning...