Yes the latest firmware is installed.Have you checked you flashed latest firmware, ie developer (weekly) 9.8.29 (2019-08-29, = MIUI 10.5)?
https://forum.xda-developers.com/t/...al-mi-pad-4-plus-clover.4178757/post-85241783
PW
The noise is not very loud but with ears close to the speakers or during night it's clearly hearable. Only solution so far is to restart the tablet. But after some time of using it the noise appears again.