A350 after powering up, high-pitched noise

After I power on the A350, regardeless if there is a tool head installed or not, something in the modules
makes high-pitched, quiet whinning that changes the frequency frin lower to higher pitched like revving engine, then lowers slowly again until goes high again. Just like some small electric engine would speed up REALLY high. Sounds almost like a camera’s flash light charging up. Keeps happening quite a long time but does not affect to the printing or engraving or to anything. Just wondering what would make that noise in those linear modules… for example if I put my ear to the X axis while the device is on but idle, there is a clear but quiet “whistling noise” like a dentist’s drill making noise somewhere. It’s hard to describe and I cannot record it as the PSU makes that much of noise under the table my phone mic cannot pick up the noise but my ears can…

Has anyone else experienced/ noticed similar?

I have heard several of the motors whine when located on a microstep. Z is the easiest to reproduce for me. Unfortunately I don’t see a way to fix it with the drivers that were chosen and their location inside each module.

I think this comes when the linearmodules do microstepping,- i had this several times but don’t bother me.
Its not loud at all, in my case.