An awful lot of wall wart power supplies have simply terrible noise problems. These are not restricted to effects pedals but can be found very often with keyboards. All of them are pretty much a simple transformer with a full wave rectifier of some sort into a way too small capacitor. The newer SMP supplies are very much better. I bought a smps designed to run a multitude of effects ( sorry I can't recall the make ATM) but I enjoy zero noise problem when I use my pedal board.
Paul
Paul