Try this too. Go into the "In" of loop one instead of the buffered input. Id doubt its a bad cable or a problem with a loop. You could try blowing some contact cleaner in the loops if its a used one...but odds are its a ground loop. The best way to track it down is to take everything out of the rack and wire it piece by piece outside of the rack(Pain in the ass...yes!). If you've got it all wired together and no noise, then try assembling it in the rack again, piece by piece until the you find the offending unit. Then you have to figure out why that unit is causing the problem....you can try lifting the ground(if its a grounded 3 prong plug) with a cheater plug temporarily, but dont use it as a fix...find the source of the issue.
Make sure if you have any walwarts powering anything, they are as far away from any audio cables as they can be. Those things will create nasty hum. I used to run a line out from an attenuator into one loop along with a series of pedals(Compressor, Phase 90, Flanger, boost pedal) and pedal order in the loops affected the noise amount greatly depending on what was placed where
Its a frustrating and irritating problem....the GCX is a great tool if you can get it to work right for you...I gave up on mine after a few years....the racked pedals were nice...but I spent more time tracking down noise issues than I cared to after a while.
Good luck with it!
