Mark Hammer's Aefea drive noise problem

[Ian]

Hi, I've built thie Aefea drive but I have a large hissing noise coming through when the pedal is on. I added a huminator board as I thought it may be mains hum but no change. Other than the hiss the pedal seems to work. Any pointers on what I could look at to get rid of the hiss please?

[Ian]