[Ham-Mac] CI-V interface noise

[EZ Rhino]

This is driving me bonkers.  There has got to be someone else on this  
list with this problem besides me.

Problem:  When I connect my iMac to my Icom 746pro (USB from computer  
to "data" port on radio), I get a LOT of noise appear in the radio.

Equipment:  PowerPC iMac, running Tiger.  MicroHam USB interface II.   
Icom 746pro.  I use MacLoggerDX and CocoaModem.

Specifics:  Click the "Radio VFO" box on MLDX, and the noise appears.  
Click it again and the noise goes away.  It is worst (unacceptable)  
on 15 meters;  also present on 20m and 17m.   Before I went to my  
current interface (MicroHam) I was using a Black Cat USB Ci-V  
interface with the same problem, but it wasn't as noisy as my current  
interface.  I mostly solved the problem before by wrapping the USB  
cable from the computer through a large toroid.  (transformer E core,  
to be precise.)  When I put the MicroHam interface on, I put an  
additional E core on the cable, and still can't reduce the noise to  
what I consider acceptable levels.  I've tried grounding the iMac,  
grounding the interface, etc. to no avail. (Where the heck are you  
supposed to ground an iMac anyways? I pulled the back cover off and  
attached a braid to a case screw.)   I am annoyed that using  
interfaces with supposed "isolation" still doesn't eliminate the  
noise.  Anyone have any suggestions for something I haven't already  
tried?

Thanks in advance,

Chris
KF7P