I have this same issue and its because the clutch deadzone wont save. I run a 60% outside deadzone so I can shift faster without grinding the gears. In order for it not to grind the clutch has to be 100% pressed before you even move the shifter and you cant let off it even a tiny bit until its in the next gear or it will grind. Super annoying I have been trying to find a fix for it since the demo.
Im using a controller and have same annoying grinding sound when shifting with manual and clutch… Every setting is on simulation no assist. Never had any problem in other forza. Its pretty annoying I hope this will get fixed asap