The other effects, treble booster, compressor, distortion, noise gate placed are before the amp input, and the eventide is in the effects loop, (Mesa Lonestar Special). This worked fine using the amps clean channel. However as soon as I engaged the Mesas dirty channel, then the Eventide caused a loud hum/buzz.
This is a classic recipe for a ground loop. Try daisy-chaining the front-end effects with a separate supply to the loop effects…or use an individually isolated supply. I would say that the problem is always there…but you only hear it with the extra gain of the dirty channel.