I have a Mesa Dual Rec that is surprisingly quiet when the guitar is plugged straight in.

However, when I go through my FX rack, the hum is louder than the guitrar signal.

I've been able to minimize it by touching the metal housing on one of the cables to the side of the rack. Obviously this isn't an acceptable solution.

The rack consists of a Furman power strip, GCX switcher, eventide eclipse, whirlewind DI, Boss line switcher, Boss Tremelo, and a Boss Tuner.

Where do I go from here?
Figure out which unit is causing the ground loop hum by process of elimination, and ground that unit somehow?
WTLTL 2011
+1 Mark

I'd play thru each unit one at a time until you identify which part of the puzzle is causing the grounding issue. Once you do that then you will have to possibly get some help using warranty's, manufacture web sites, manufacturer specific forums, schematics, friends, bumps, etc to figure out how to make sure that unit is grounded properly. The buzz goes away when you touch the metal rack because you are completing the earth ground.
I've done this and it seams just going through the GCX with out anything in the loop is causing the buzz.

so what now? do I have to take the damn thing apart?

Has anyone else had grounding issues with the GCX?
if you are not worried about warranty's then I guess I would open it up and try to find the ground. Maybe it is loose.

I realise your unit is prolly sealed pretty tight. If you have a warranty then exercise that.

Try pluggin it into one of you other known good rack modules and see if it keep appearing?

sorry im not an expert on this
Touching it resolves the issue I'm assuming? If thats the case you could just run a metal wire from that to a different grounded unit or connection. Or like 311 said, figure out the internal ground loop issue and fix it, or make a new ground.
WTLTL 2011