[Kenwood] Slight Hum on CW TS-940S

[Dennis Younker NE6I]

It's been pointed out to me that I have some hum on CW transmit on my 
TS-940S.  I listened for myself using another transceiver set up nearby. 
Sure enough, I can hear the hum. It's present regardless of band or power 
level. It's difficult for me to tell whether it is present on SSB. Is there 
a part that is most likely to cause this problem in this rig?

--Dennis, NE6I