I've had a couple of DE that hum...my Hook though, is silent.
I've replaced the suggested caps and tightened screws, added jumper ground wires, etc.
Nothing really fixes it as far as I can tell. Clay says to change the caps, but in my experience that doesn't really work well; maybe a 5% reduction in the hum, which isn't very much.
Someone wrote one that it's a problem with the dirty power out of the transformer, when I emailed him for more info, he didn't respond.
Also, I found someone that had a friend that built a daughter board for the sound board (I think) that fixes the problem. But he won't build any more and wouldn't give me the schematics or any other info so I could do it myself.
So much for this being a small nice "club".
I took the problem to one of the electronic engineers here, and asked him. He thought it was a design issue with driving some of the amps too hard, combined with poor quality (large variations in batches) used and 60 or 120 cycle noise. I put a scope on the line and saw a lot of 60/120 hz noise.
I just sold both games before I figured out a solution.
So a bad regulator might be in the realm of possibilities. As well as a lot of other things that create noise in the circuit.