hum problem, rack rehousing

[Tav]

I have reahoused 4 stompbox effects in a rack enclosure, a Flanger Electric Mistress, a Boss Ce2, Boss Tremolo and Phase 90.
I'm using the new unit with  a Cae switcher, using 2 loops for the 4 effects, the chorus and flanger share the same input and output, as well as the tremolo and phase, and I toggle manually between them with a small dpdt on on mini switch on the front of the rack. It seems that the flanger has a little ground hum that affects also the chorus, due probably to the non perfect isolation of the selector switch. How can I isolate completely the 2 units? Must I filter in some way the inputs or outputs and dpdt switch?
Regards