I had a similar problem. Try switching to a patch where you can audibly hear the second oscillator. Adjust the second oscillator's level in the parameter section. Can you hear the drone sound being affected by adjusting the second oscillator's volume?
I was about the adjust the second oscillator with an oscilloscope per the manual and noticed that a 4051 ic was burnt. I replaced this chip, restarted and the drone sound was gone.
So there's two possibilities.
Good luck.