@nikscha The Duet 2 WiFi is set to spreadCycle continuously. You could try running it in spreadCycle on the Mini 5+, and to avoid coil whine have it switch to stealthChop when stationary (set changeover speed at below the jerk value). The spreadCycle on the Mini 5+ is different from the Duet 2; they use different drivers, and different versions of spreadCycle.
Are the other axes noisy? What happens if you swap motors over?
Ian