MarkT:
The thing you must avoid is powering the relay coils from the same supply as the audio circuitry - entirely
separate power and ground harnesses are required, and if you need to share grounds do so at one point only so
no ground loops are created.
Yes. I forgot to mention that ground loops are a problem in other places than the studio. If a PCB is not laid out properly, ground loops can occur there, too. See my previous discussion on eliminating noise, where I discuss this very issue (I just forgot to use the term "Ground Loop") -- also, my schematics, where I attempt to illustrate the cause of noise injection due to wire resistance, feature the "High Side" [i.e. the positive power lines]. But, the same principle causes this on the ground side. And the method I prescribed for preventing this, is the same for the ground side.
Also, I'm beginning to think the audible ticking sound and the noise present in your audio signal, are unrelated. Or, if they are related, one is not causing the other. They are each being caused by the same other thing [i.e. something that is all together different]. For instance, a current surge [like would be caused by energizing a relay coil], could be, both causing a capacitor to "tick" and could be causing the Audio Signal noise by some sort of coupling [capacitive, magnetic, electromagnetic transmission, or merely a ground loop, etc.]