[Elecraft] Hum in the Sig mode-KX1

[SJH]

I have completed the KX1 including the 30M option and have not yet added
the ATU module but noticed I here a 60 cycle hum in the audio when I
switch to the Sig mode which shows the second digit blinking indicating
this is in the Bar graph mode.   When I switch out of this by pushing in
combination the  Band and Rit buttons , it goes away.....Any
suggestions??????  Transmitter section seems to check out at this point
and I don't seem to have any shorts in the 30M module

Steve
KQ4UB