A few questions:
Does this happen with every H90 program or only specific ones?
Are you using factory default settings or have you changed the operating levels of the H90 for any reason?
Can you trying using any of the INIT programs on the User 2 list? These consist of two THRU algorithms.
Is the noise still present when you bypass the H90?
What kind of a guitar are you using; does it have single coil pickups?
Is the H90 connected to a different power outlet than your amp is?
