3 of my 6 kdk had this problem. besides humming noise, fan speed also affected. 3 speeds became 2. Speed 1 was ok, speed 2 was about 70% what it used to be and that was the fastest it could go. fan will beep pressing 3 on the remote but speed didn't increase.

for me, changing capacitors fixed the problem on those 3 fans. my fan pcb has 5 caps and a set of them costs around RM 10 on shopee. another option is to change the entire pcb if you don't want to solder but each board costs around RM 40 - 80 depending models.

hope this helps.