Hi @gabornico,
If you see the attached picture, you could find the memory modules of Samsung and Micron were not mixed in the mainboard of Z640. The 1st case is 2x Samsung 32GB 4DRx4 PC4-2133P-L Low Loaded Module (M303A4K40B0BB0-CPB) and the 2nd case is 4x Micron 32GB 2Rx4 PC4-2400T-R ECC Reg RAM (36ASF4G72PZ-2G3B1).
I think that the available BIOS of Z640 maybe has a bug to match the rotating speed of two rear fans when 32GB modules are installed on the mainboard and result in the higher rotating speed of 2200 RPM. But the BIOS of Z840 workstation has no problem to match 32GB DDR4 modules.
