Humming effects -
Does the effect
Effects with AC power supply built in
These effects are prone to the normal problems that AC powered equipment can have with hum.
Always used with an external wall-wart adapter or power supply
It may not be the effect at all. Wall warts often leak AC hum, or have poorly filtered outputs. Try it with a wall wart known to be quiet with other effects.
Always powered by a battery
It's probably got a loose ground connection inside the pedal that is not tying the effect signal grounds together throughout the box. Open it up and check for broken wires to jacks, pots, the effect board, and so on. It is also possible that one or the other jack in the effect has a dirty or contaminated ground ring, and the signal cord's not making good contact; or that the jack is cracked a bit. If this is a new pedal you've built, it is possible that all the grounds are not connected together on the board or that a bad solder joint is keeping it all from making contact.
work without hum by itself but hum when it's put into a pedalboard?
It may not be a problem with the effect itself. If it only hums when put into a pedalboard, it is likely that the grounding or power supply on the pedalboard may be causing ground or AC hum. It is also possible that it's sensitive to magnetic fields, and the pedalboard may have an AC power supply that is broadcasting magnetic leakage right where this thing is sitting.
Pedalboard hum can be defeated by two methods:
(1) breaking the common ground line between effects (ground loop) that is caused by sharing a common power supply ground. The Spyder pedalboard power supply shows how you might do this and explains what is happening.
(2) break the ground loop by removing the ground to ONE end of every interconnecting cable between two effects on your pedalboard
(3) adding an active ground isolator to each effect