K3 Line in - data mode only problem - hum

[k2pi]

Hi:

I've just discovered (on a RTTY contest weekend of course) that my K3 has a terrible transmitted hum when keyed from the PC using the line-in jack, but only when I am switched into data mode.  If I switch to LSB, STILL USING THE LINE-IN JACK FOR AUDIO, this problem disappears completely.  

I just go the rig back from Elecraft this week, and it had a major repair, but also a complete going-over.  I doubt the problem is in the rig.  There must be a setting somewhere that is causing this when I switch to data mode, but darned if I can find it  Any help appreciated.  

Thanks and 73,

Harv
K2PI