Scratchy switches... daisy chain problem ?

Hi!
I've got a little problem which seems to be general, for all my pedals. When I switch then on and off, I hear a little scratchy sound, like a slight buzzing. Having the same problem with 8 pedals at the same time makes me think it's not the switches. I do daisy chain a few of them (but not all of them). Could it be a problem with the adaptor I'm using or should I look elsewhere?
 
Process of elimination. Try each pedal by itself, not connected to any other pedals or shared supplies. Then try them in groups of two or three. Try them with one pedal on one power supply and another pedal on a different supply, or on battery. Try the pedals in different order in the chain. In each of these steps, including the way you already have it set up, does the scratch/buzz sound change in volume or quality if you step on the switch a few times in quick succession, or let it go for several minutes before hitting the switch?