high pitch sound from T1

[Joel65201]

I am running a Zoom G3n into a T1 and on to a Compact.  Sounds find.  If I put anything in front of the G3n--another G3n, an EQ, etc--there is a high pitched done that comes through.  This does not happen if I go from the G3n directly to the Compact.  It is not an ear-splitting sound--but definitely audible to an audience.  The music coming through the Compact sounds fine (other than that tone).

I am using unbalanced patch cables (I have tried several--and I have never used anything but unbalanced cables).  I have by-passed ToneMatch, eq, effects, etc., on the T1 changed power outlets (used a Furman) all with no change.  Again, if I bypass the T1, it is fine.

Can't help but think I am missing something.