mhuss

I have a medium large pedal board with a few 'dirt' pedals, and which had a Line6 M5 for the rarely used effects such as rotating speaker. All the pedals use an isolated output 9v supply, and hum and noise is at very reasonable levels, all things considered. However, when I put the M5 in line (using its own independent power supply), I get an awful(!) hum from the gain pedals when they are engaged. If I disconnect the M5 power, the hum goes away (M5 still inline with the other pedals). I tried two different supplies for the M5, with no apparent difference in noise level, and the hum is there even with the M5 in idle/bypass mode - it just has to be powered up. I'm scratching my head wondering what might cause this. Any ideas?