I have the 4 board DK PCB setup. I am having this same problem that I found on Mikes Arcade.com
Problem: Whirring sound all the time.
Sounds like the jump sound never completely cuts off.
Solution: replace transistor Q4 on the CPU board, a 2SC 1815
I don't see anywhere on the PCB Q-4 letters don't go that hugh. I didn't see anything like this on the monitor board either. Anyone know where this is?